Add subcommands for common operations

[rvantonder]

The default positional and anonymous arguments make it challenging to use comby for distinct operations like:

• rewriting a single pattern
• just matching files
• using a configuration file that contains multiple patterns

Subcommands for these operations will help reduce command line trickiness. Currently, the idea is to support:

• comby find ...
• comby rewrite ...
• comby config ...