Implement macros

[peterhinch]

As per issue #111

[peterhinch]

@dhylands In response to your suggestion of a default macro file this is now implemented. Its name is rshell_macros.py and it should be located on the Python path. The contents of any user-specified macro file are added to those in any default. This is done by means of dict.update. Consequently in the case of identical macro names the specified file has priority.

I have not adopted your suggestion of ~/.rshellrc because macro files comprise Python source and should therefore conform to Python naming and search conventions. I would suggest that users of the macro facility will be experienced Python coders who will be familiar with and expect such conformance.