Add option to exclude certain folders and files

[swissspidy]

The make-pot command currently scans all files with a .php extension (or .js, see #26).

In many cases, there might be files and folders that shouldn't be scanned. Some examples:

• node_modules/ (npm modules)
• Gruntfile.js
• webpack.config.js
• vendor/ (Composer dependencies)
• .git, .svn, .svn -> basically anything with a dot prefix

Being able to exclude these files would increase performance and create less POT files with potentially unwanted strings in it.

Perhaps there could be even a default list of folders and files to include.