use: switch form PATH to fish_user_paths

This might impact a few people, but I think this approach is correct as
it prepends, but also gives you flexibility to manipulate this
prepending part if you don't want ruby path to be very first amongst
these.

An upcoming idea is to allow users to configure if it should be
prepended or appended to the `fish_user_paths`. And also put in some
bundler support with binstubs.

Basically integrate this:

https://github.com/terlar/dotfiles/tree/f6a132761a25a5ccda429dafbf0fa1dc17376797/config/fish/plugins/bundler
config: test current configuration output
config: add support for prepend path config

You can now choose if you want to prepend or append the ruby when you
switch. Please note that it is prepending or appending to the
`fish_user_paths` which means it will be before anything else in the
regular path even if you append. This is just to allow you control your
own user added paths and the order of them.
Merge pull request #30 from terlar/feature/use-fish-user-paths

use: switch form PATH to fish_user_paths