In order to pro-actively diagnose an issue on someone's computer, a
`omf doctor' command is being added. This command will initially check
errors that could cause a theme to not be loaded, but in the future more
checks are going to be added to prevent issues from being created for
problems that are fixable by the Oh My Fish user alone.
Moving and renaming install_fish.sh to travis_install_fish
Moving travis_* from tests to tools.
rename: fish_user_function/ -> functions_override/
renaming travis_test.fish to test_runner.fish