Adds a menu for grabbing input from bin/fragment-*.  Adds a couple of
new fragments, and moves today → fragment-today.

This needs work:

  - it's remarkably slow
  - it has an external dependency in FZF
  - I'm not sure about the keybindings
  - it removes a couple of bindings I'm really used to
  - it's not documented with "CUSTOMIZE" markers
  - this only works in gvim with a recent build of vim 8.1

It might be better if the fuzzy-finder were vim-native, if it
distinguished between full-on document templates and small fragments, etc.

I'm also starting to think it'd be good to compartmentalize some of this
config.

Things were getting a little unruly in there.

- bunch of comments
- adds an F6 binding for opening the most-recently used file list in
  MRU
- changed the behavior of `today`; it no longer outputs HTML tags
  on its own.