Adding .get methods

[henryiii]

This was originally going to be a fix for #193, but it turned out that it was already fixed in 1.5.0. However, this is still a more intuitive and useful way to try multiple commands, imo.

Given the method of #193, you currently can do

gzip_cmd = getattr(pb.cmd, 'pigz', pb.cmd.gzip)

Now you can do

gzip_cmd = local.get('pigz', 'gzip')

This is reminiscent of the .get function on dicts. This also can be used to search common locations for commands, for example,

pandoc = local.get('pandoc',
                   '~/AppData/Local/Pandoc/pandoc.exe',
                   '/Program Files/Pandoc/pandoc.exe',
                   '/Program Files (x86)/Pandoc/pandoc.exe')

since it returns CommandNotFound if the path is invalid.