Fix variable name in doc
Grammar tweaks
Doc edits and rename three functions

Change: TLet g:vikitasks#sources = {...
Reason: Keep on one line for generated docs to show default.

Change:
s/vikitasks#ItemMarkDueInDays/vikitasks#ItemsMarkDueInDays/
s/vikitasks#ItemMarkDueInWeeks/vikitasks#ItemsMarkDueInWeeks/
s/vikitasks#MarkItemDuInDays/vikitasks#ItemMarkDueInDays/

Reason:
- name functions that deal with many items with the plural
- MarkItemDuInDays was misspelled; while fixing that change to follow the same
  pattern as the plural functions.
Merge branch 'master' of github.com:jean/vikitasks_vim

Conflicts:
	autoload/vikitasks.vim
	doc/vikitasks.txt
	plugin/vikitasks.vim