Allow different "generate" and "use" chars

[hlascelles]

We would like to generate thousands of alphanumeric links like this:

foo.com/1Gk82
foo.com/MB8d3
...

However sometimes we also would like to make a "permashortlink"

foo.com/flash_sale

As you can see flash_sale has an underscore. When it goes through extract_token it comes out as flash and isn't found.

I understand why this is (to stop a foo.com/1234. lookup looking for 1234. with a fullstop, right?). Would you be open to a change to have extra config to have both "chars used to generate randomly" and "chars to look for in a path"?

[craigmcnamara]

Send a pull request if you'd like to see this