config files

There are too many arguments to pass to `Qdb` to make setting breakpoints fast and simple.

This adds a `.qdb` file parser and scheme. The `QdbConfig` is a namedtuple that holds all the arguments to be passed to Qdb.

It checks for config files in:
- cwd
- `$HOME`
- Any files explicitly given.

`QdbConfig`s can also be constructed from a dict to make setting custom configs programatically simple.