Add allow_unicode=True to slugify

[tcely]

I haven't given this change great consideration.
If there is a reason not to allow Unicode, then please let me know.

@meeb Why are underscores disallowed?

Fixes #527

[meeb]

Only reason underscores were disallowed in titles was to make the default media template slightly easier to parse as it was originally date_channel_sluggified-title.ext. Allowing underscores in the sluggified title made it slightly messier. This is back from the days when this was just my personal downloading tool. I'm fine with allowing underscores if there's a request for it.

The Unicode issue was for people storing media on some network shares (and possibly Windows?) which caused the file names to either error when writing certain characters or write files with the ever popular black squares all over the file name. This again was from a few years ago and might not be an issue now.

Edit: I thought Unicode was already allowed, obviously just implemented badly so there were some problems as described in the linked issue.

[tcely]

I'm fine with allowing underscores if there's a request for it.

Ok. I'll request and implement that change then.