Configure additional invalid characters

[Chaphasilor]

Hi, I'm using the module for sanitizing file names for downloaded video files, and process those files later on using FFmpeg. The problem is, your module deems a ' as safe, but it will cause an error if used with FFmpeg.
There might be workarounds for that, or I could manually remove '-characters, but I'd prefer if you could offer us the option to specify our own reserved characters, either in addition to the existing ones or as a replacement to them.

For example, have and option to call sanitize.(<input>, {invalid: ["'"], replacement: "_"} and have it replace everything it already replaces, but also '.

This shouldn't be too much work but I think it would be very useful!

[parshap]

Somewhat related: #11

Your suggestion makes sense. I'd be happy to accept a PR that implements this.

[Chaphasilor]

Hah, I thought you'd say this :D

I'll see what I can do...