Allow custom charsets
Allow custom charsets

This would allow you to easily override the charset with whatever you wanted to allow, like allowing dashes for instance:

In initializers/shortener.rb you could then have something like:
```
Shortener.charset = ("a".."z").to_a + ("A".."Z").to_a + (0..9).to_a + ["-", "_"]
```
jpmcgrath/shortener,jpmcgrath/shortener