Rewrite _{append,prepend}_path, delete _increment/_deduplicate

Rewrite _append_path and _prepend_path to work better, be able to modify
variables from their caller, and handle deduplication inline instead of
using _deduplicate.

Remove _increment, as it was only ever used from _deduplicate and wasn't
actually doing anything useful there.

Remove _deduplicate, as it was only ever called from _append_path and
_prepend_path.
Rename _fish_* functions to __tacklebox_*

Also clean up the function path handling. There may be more user
function paths than just the first one in $fish_function_paths.
Rename all the fish_* settings to tacklebox_*

Also remove _append_path and _prepend_path, instead define
__tacklebox_prepend_path in tacklebox.fish directly.

Remove the need to put tacklebox in your tacklebox_path. Since it no
longer ships with any functions (or depends on said functions), it's
only necessary if the end-user stores things there.

Only load the theme if tacklebox_theme is actually set.

Add .gitkeep modules so a fresh checkout of tacklebox gets the expected
modules/plugins/themes folders.
Merge pull request #1 from kballard/cleanup_tacklebox

Clean up Tacklebox