New command: ignore

[DavideRossi]

Add a new ignore command (upback ignore file(s)) so that the user can specify them without manually eiditing .upback.config. How to handle wildcards? Maybe wildcards should be supported but should be extended before adding the ignore (so it is not possible to specify an ignore with a wildcard). Notice: Window's cms does not perform wildcard expansion so the glob module should be used from Python to perform expansion.