Add Possibility to fold on context or project

Closes #30

Folds now works the following way:

+ At initialisation, if all completed tasks are at the end, folds are
based on completed tasks only, else they are based on context
+ After a hierarchichal sort, folds are set by the first level between project and context
+ After a "normal" sort, folds are set only on completed tasks (as
before)

This work is greatly inspired from the snipped provided by @arecarn

autoload/todo.vim

@@ -150,6 +150,7 @@ endfunction
 function! todo#Sort()
     " vim :sort is usually stable
     " we sort first on contexts, then on projects and then on priority
+    let g:Todo_fold_char='x'
     if expand('%')=~'[Dd]one.*.txt'
         " FIXME: Put some unit tests around this, and fix case sensitivity if ignorecase is set.
         silent! %s/\(x\s*\d\{4}\)-\(\d\{2}\)-\(\d\{2}\)/\1\2\3/g
@@ -268,6 +269,7 @@ function! todo#HierarchicalSort(symbol, symbolsub, dolastsort)
         "Empty buffer do nothing
         return
     endif
+    let g:Todo_fold_char=a:symbol
     "if the sort modes doesn't start by '!' it must start with a space
     let l:sortmode=Todo_txt_InsertSpaceIfNeeded(g:Todo_txt_first_level_sort_mode)
     let l:sortmodesub=Todo_txt_InsertSpaceIfNeeded(g:Todo_txt_second_level_sort_mode)

ftplugin/todo.vim

@@ -106,26 +106,57 @@ setlocal foldmethod=expr
 setlocal foldexpr=TodoFoldLevel(v:lnum)
 setlocal foldtext=TodoFoldText()
 
+" Go to first completed task
+let oldpos=getcurpos()
+let g:Todo_fold_char='@'
+let base_pos=search('^x\s', 'ce')
+" Get next completed task
+let first_incomplete = search('^\s*[^<x\s>]')
+if (first_incomplete < base_pos)
+    " Check if all tasks from
+    let g:Todo_fold_char='x'
+else
+    " TODO detect if sorted on prjects
+    let g:Todo_fold_char='@'
+endif
+call setpos('.', oldpos)
+
+function! s:get_contextproject(line) abort "{{{2
+    return matchstr(getline(a:line), g:Todo_fold_char.'[^ ]\+')
+endfunction "}}}3
+
 " TodoFoldLevel(lnum) {{{2
 function! TodoFoldLevel(lnum)
-    " The match function returns the index of the matching pattern or -1 if
-    " the pattern doesn't match. In this case, we always try to match a
-    " completed task from the beginning of the line so that the matching
-    " function will always return -1 if the pattern doesn't match or 0 if the
-    " pattern matches. Incrementing by one the value returned by the matching
-    " function we will return 1 for the completed tasks (they will be at the
-    " first folding level) while for the other lines 0 will be returned,
-    " indicating that they do not fold.
+    let this_context = s:get_contextproject(a:lnum)
+    let next_context = s:get_contextproject(a:lnum - 1)
+
+    if g:Todo_fold_char == 'x'
+        " fold on cmpleted task
         return  match(getline(a:lnum),'\C^x\s') + 1
+    endif
+
+    let fold_level = 0
+
+    if this_context ==# next_context
+        let fold_level = '1'
+    else
+        let fold_level = '>1'
+    endif
+
+    return fold_level
 endfunction
 
 " TodoFoldText() {{{2
 function! TodoFoldText()
+    let this_context = s:get_contextproject(v:foldstart)
+    if g:Todo_fold_char == 'x'
+        let this_context = 'Completed tasks'
+    endif
     " The text displayed at the fold is formatted as '+- N Completed tasks'
     " where N is the number of lines folded.
     return '+' . v:folddashes . ' '
                 \ . (v:foldend - v:foldstart + 1)
-                \ . ' Completed tasks '
+                \ .' '. this_context
 endfunction
 
 " Restore context {{{1