1. Fix insert timestamp failure problem for zh-tw
2. Integrate calendar.vim plugin to implement date picker function
Documentation for inserting timestamp using calendar selection
Add syntax highlight for breaking down subtasks.
Add error message for "unable to activate plugin"
Add function for getting dom obj start and end line number.
A base class for abtraction of dom obj, like Heading and Checkbox.
Refactor the "find_heading" function based on get_domobj_range.
Refactor headings.py and add checkboxes.py

 - Refactor Heading and HeadingList class by using DomObj and DomObjList
   from dom_obj.py
 - Add Checkbox and CheckboxList class for checkbox functions
Add EditCheckbox plugin for checkbox function
Write checkbox object to vimbuffer
vim plugin config for checkbox function

 - Add EditCheckbox plugin
 - set expandtab and tab spaces configs
Add test cases for new functions

 - test_libbase.py for testing base.py
 - test_libcheckbox.py for testing checkboxes.py
 - test_plugin_edit_checkbox.py for testing EditCheckbox.py
Toggle checkbox when not finding checkbox
Merge remote-tracking branch 'upstream/master'

Conflicts:
	ftplugin/org.vim
	ftplugin/orgmode/_vim.py
Refactor EditCheckbox and fix checkbox toggling without title

- checkbox toggling will work fine if no title for checkbox
- Refactor toggle and update_checkboxes_status function in EditCheckbox
  plugin
Clean up messy code and correct init_checkboxes behavior

- Remove unnecessay comment
- Limit init_checkboxes range in current_heading
Insert new checkbox for EditCheckbox plugin

- Insert new checkbox above/below functions
Add update subtask status for checkbox title
Fix init_checkboxes boundary error

- If checkbox range is out of heading end line, checkbox end should be
  heading end line
Support checkbox indentation