Ability to ignore errors

[martinohanlon]

Errors being raised are not always desirable in scripted situations, an option to ignore errors is required.

Based on requests in #7 from @kiwi0fruit

[martinohanlon]

Add the ability to create ShortCutter in silent mode by adding silent to the constructor.

silent would suppress any exceptions being raised.

Also add --silent to shortcut application.

[kiwi0fruit]

I suggest explicit skip_errors=True/False (same but inverted is raise_errors) and error_log=None/sys.stderr/io.StringIO() where None means silent mode. This way skipping errors cannot be activated in two different ways (as you suggest).