local_cmmds.tex: Add \iref, \fref, and \xref

local_cmmds.tex: Add \iref, \fref, and \xref

This makes it easy to get consistent (and flexible) formatting for
references to external equations, figures, etc.  I'm surprised that
this isn't somehow bundled into cleveref or natbib.  If I get time, I
may try and pull it out into its own package.

Based Joseph Wright's answer [2] to a question about switch statements
[1].

[1]: http://tex.stackexchange.com/questions/29133/how-to-create-switch-structure-comparing-strings-in-latex
[2]: http://tex.stackexchange.com/a/29139