set &formatprg to fish_indent

This makes it possible to fix indentation and prettify code with fish's
own formatter using gq commands such as gqip to format a paragraph.
use fish's man function for &keywordprg

This makes it possible to use K to view the man page for the fish
builtin under the cursor.
support for the matchit addon

This makes it possible to use % to cycle between the start of a control
structure and its 'end', among other things the matchit addon is capable
of.