Only show deprecation warnings when omf is loaded.

Fix error handling when github API is unavailable.

The `exit` (instead of `return`) caused the shell to quit if there was
an error with github.

README: add title and foreign-env plugins

The oh-my-fish.fish startup file is sourced when changing themes after
the `fish_theme` variable is set.  The deprecation warning should only
be shown if the user sets the deprecated variables.

Add omf commands for listing available plugins/themes and for downloading a plugin/theme

fixes #496 Do not throw warnings when theme is not set

Fixed typo in CONTRIBUTING.md

Updated link to theme switcher plugin

Old link was 404

Fix outdated project information

Modify plugins README.md

Added plugin-grc.

fix issue #474 by using pushd/popd

… since it's likely that some plugin authors will simply copy that one, let's make it a link :)