Add new option "extensions"

By default, directory-encoder encodes all `.svg` and `.png` files in the input directory. With this option you can tell directory-encoder to only encode files with specific filename extensions (e.g. by setting `extensions = [".svg"]`). The default stays `extensions = [".svg", ".png"]`.

Motivation: I have a folder that contains `.svg` files and corresponding `.png` files with identical filenames (except for the filename extension, e.g. `file.svg` and `file.png`). Without this change the `_checkName` method aborts with the error that two files with the same name exist in the input directory.

(Alternatively this could be implemented using a regex match on the filename extension or the whole filename. Please tell me if you'd prefer this or another approach.)