Adds a filter to allow linebreaks on a forward slash

Not sure how universally useful this is, but I found myself needing it for a project. Figured I'd add it to typogrify rather than building my own plugin and put it out there in the event anyone else needs it.

The breakslash filter inserts `<wbr>` after any forward slash not preceded by `<` (which should prevent it from messing up HTML tags). `<wbr>` allows the browser to break the line at this point. Compensates for browser inconsistencies with the CSS `word-wrap` property, at least in this one tiny case.