Install readtags command by default

Hello,

These changes add support for installing the readtags command, which is useful in its own right to access tag informations from the shell.

readtags is not a very well known part of exuberant-ctags, and is a good quality command line tool, I think it would make sense to install it by default so that other tools could start relying on it rather than using the readtags C library or (even worse) by manually parsing the tags file.

Cheers,

Maxime Coste.