bmessage: use regex breakpoint to match category

Resolves #57

With this, commands like:

```
bmessage [XYZCustomView setFrame:]
```

Will correctly set a breakpoint on `-[UIView(Geometry) setFrame:]`.

Updated:

This pull request now handles explicit categories on input. For example:

```
bmessage [XYZCustomView(Geometry) setFrame:]
```

Now works where previously it would not work. Additionally, if the category is given, then the breakpoint is created with the `--fullname` argument instead of `--func-regex`.