Add command line utility for editing cassettes

[sirex]

When you have large response or request bodies it is almoust imposible to edit then directly in YAML file.

To fix that, I created a command line tool, that will open request or response body for editing in an external editor and when you save you changes, cassette file will be updated with you changes.

This saves a lot of time and prevents errors when trying to edit large responses directly in YAML file.

Teststed this on py27,py34,py35, all tests pass.

Added new section in documentaiton about how to use command line tool.

Probably this fixes #249.

[neozenith]

Is there still a need for this PR? It has been 2.5 years and the library has moved on a bit. (Sorry, accidentally hit comment and close).