feat(agenda): Add custom agenda commands

[kristijanhusak]

Add custom agenda commands.
Closes #478 and #135 .
Check org_agenda_custom_commands in the docs for more details.

Also fixes few other issues:

1. Filter by todo state in agenda view #849
2. Hide tags from headings #776

Few more notable features/bug fixes:

1. Agenda now renders any markup that is part of headline (bold, italic, date, link). Links are also clickable for Neovim 0.10.2 + due to OSC 8 control sequence.
2. Doing insert link mapping on the link was broken by not prompting to update the link under cursor. That is now fixed.
3. Unrelated to agenda, emacs export is updated to attempt to find a configuration file, and fallback to no file in case of an error