Fix for <Leader>x and other minor issues

Fixed a bug in <Leader>x (todo#PrependDate) introduced last commit.
Fixed case sensitivity bug in todo#ToggleMarkAsDone() and todo#UnMarkAsDone()
Fixed errors being reported for repeat#set if vim-repeat plugin is not installed.
Fixed modeline in .vim files to work (only works on first/last 5 lines of file), and made the modelines consistent across all files.
New unit tests for todo#ToggleMarkAsDone()
Minor fixes for README.md and man page
Fix anchoring of RegExp in todo#ToggleMarkAsDone
Fixed a bug in overdue highlighting that resulted in the current date being matched on round dates like the 20th and 30th.
Added more unit tests for overdue date highlighting.
Corrected RegExp anchoring on today date highlighting.
Added a few bash scripts to make running unit tests easier and more reliable, including testing with ignorecase user preference set.

tests/clean-vim.sh

@@ -0,0 +1,16 @@
+#/bin/bash
+
+# Start a clean vim for testing development.
+
+REPO_TOP=$(git rev-parse --show-toplevel)
+cd "${REPO_TOP}"
+
+vim -Nu <(cat <<EOF
+filetype off
+set rtp+=~/.vim/bundle/vader.vim
+set rtp+=./
+filetype plugin indent on
+syntax enable
+EOF
+) tests/todo.txt
+