Colorize status output & tone down logging

 - I've removed a couple of log lines, e.g "switching" & "reloading". I think the "loading .envrc" line is plenty of notice about thode actions.
 - only log the "export VAR" line when there's something to log

In the second commit, I added colors. Currently I'm using grey for the status lines, and red for any errors. This greatly helps me notice when there's an issue (i.e. I need to run `direnv allow`), while visually ignoring the normal status lines (which I rarely need to read).

Configuration might be necessary to control the colors on/off though, and perhaps others want to use different colors. Thoughts?