New features: Support for week format specifier & additional default autosuggests

[gillesgagniard]

This pull request implements #93, and adds the following features:

1. A support for "week:" format prefix, similar to existing "time:", but applying a week format configurable in settings (use case is typically inserting current week number, next week number, etc ...). If dates are configured to be inserted as links in the settings, this PR allow for creating weekly links.
2. Additional commands for parsing and inserting date formatted as weeks
3. A new setting to configure the default autosuggest popup contents with additional entries (use case is customizing it to add "week:this week" or "time:now" for instance)

[gillesgagniard]

This PR replaces #102 in which old commits had been pulled in for an unknown reason.

[gillesgagniard]

Hello ! Any thing I can help with for merging this PR ?