tags: strip or replace unsupported delimiters

[unsystemizer]

If I create tags like a, b, c, I'll mess up a and b because they'll have the comma attached to them.

If somebody uses an unsupported delimiter, it could be automatically replaced by the space character.
In that case and , and ; could all be accepted and that could be indicated in the hint above the tags form field.

[sebastian-code]

Thanks @unsystemizer. As I said in my comment on #74 Actually I have been thinking about it, thinking to use Taggit instead. This obviously will fix #74 too. Let work it a little bit more, or, if you have suggestions or perhaps if you submit a PR for this, it will be most appreciated.

[sebastian-code]

Solve with 65aabe8