Add extension filter

Add an extra argument to [cli struct](https://github.com/KarlHeitmann/rg_explorer/blob/be5ef20afbc107dbc5cb38414c534f59f38314c0/src/main.rs#L18), so you can add a list of extensions. rip grep will perform the search only in the file extenions provided by this list.

Example:

> cargo run -- div --include_extensions "html css"

This will perform a search for "div" pattern only in the *.html and *.css files on the current directory.

I think there is a `rg` option that lets you perform the search in some type of files. This would be the best solution.

Another solution would be to perform the search inside all files, and then in the "Nodes" tab apply the extension filter to the results.