Toggle between creation and modification date of item

[theg4sh]

This PR's fixup a bug when date could be appended twice or more times.
To reproduce the bug edit an existing item with pressing key few times.

Behavior which date should be stored for item is configurable.
let g:VimTodoListsDatesKeepCreation = 1 (or any value not eq to 0) to keep the date of item creation
when the value of config is equal to 0 (zero) timestamp of item will be overwriten with current timestamp.

[aserebryakov]

@theg4sh, can you please describe the issue with a couple steps? I couldn't reproduce it.

[theg4sh]

@aserebryakov oh, sorry for long response time, here is command to reproduce the issue:

$ vim /tmp/test.todo
:execute "normal i\<cr>first\<cr>\<esc>dd3Ea second\<cr>\<esc>dd4Ea third\<cr>"

The result will be the following:

 (18:01:11, 01 sep 2020)
- [ ] first second third (18:01:11, 01 sep 2020) (18:01:11, 01 sep 2020) (18:01:11, 01 sep 2020)

With option in that PR you could toggle between line's creation time and modify time.