Show manpage for command under cursor in __fish_man_page

[newsch]

Description

This lets you check the manpage for a leading command by moving
the cursor over it, matching the behavior of tab complete.

It also lets you select the man page for the base of a two-part command
like string match.

I added the additional regex case because
commandline -t returns an empty string when the cursor is after a
space, e.g. at the end of 'sudo ', which the later checks don't handle.

This diagram shows the manpage picked for different cursor positions:

> sudo -Es time git commit -m foo
  +-------++---++--++------------+
  |       ||   ||  ||            |
  |       ||   ||  |+------------+
  |       ||   ||  |  git-commit
  |       ||   |+--+
  |       ||   | git
  |       |+---+
  |       | time
  +-------+
     sudo

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[krobelus]

great change, thanks!