.slugify: added optional second argument for customisable separator

hey,
needed a slug with underscores instead of dashes as a separator in one of my projects.
added a second optional argument to .slugify to make it more customisable.

realising the call that replaces .dasherize looks very ugly and is not perfect. as soon as i get some feedack on whether you agree on the customisability of .slugify, i'd suggest writing an equally customisable .dasherize (would need a good name for that ;)

what do you think?