Crayons

* Check that `sys.stdout` object has `isatty` attribute in addition to it being `True`
* If using a custom object that does not have the `isatty` attribute and you would still like color, you can set the `CLINT_FORCE_COLOR` environment variable

Bugfixes

* Explicitly set incoming parameter to crayons to string (so that len, etc. work as expected)

__Breaking__:
* Rewrites `clean` to just remove ANSI codes, not whatever it was doing before (which seemed to be stripping all characters from the string)

__Features__:
* Adds `random` function to get a random color (thanks azillion)
* Adds `replace_color` function to allow setting replacement for a given color (e.g. replace blue with magneta) (thanks frostming)
* Adds `reset_replace_color` to reset any replacement colors set