Allow filter on variables list

With non trivial script, variables list becomes a huge list, making it somewhat unmanageable. Sorting by most recent update (another open [issue](https://github.com/inducer/pudb/issues/107)) would do help but It would be cool to allow **"/"** key to start a filter on variables list

Here you are some quick thoughs:

- use * as regex .*
- use , to search multiple patterns
- use . to search on second level variables (nested)

examples:

- first,last

Show variables **first** and **last**
- current* 

Show variables like **currentLine**, **currentColumn**, **currentFile**...
- element.pos,.size

Show variable **pos** inside **variable** element, an every **size** variable inside whatever variable