Cleaning up the brush command

[nlipiarski]

This pull request provides some clean up to the parsing of the brush command's arguments. The changes include:

• Making the brush size argument explicit, but still optional (i.e. shows up in the usage)
• Added a dedicated Brush command element for parsing the command line brush arguments
• Removing performer consideration from the brush command since we also have the performer command. I think this makes the brush commander more intuitive and also makes the code cleaner.