Caching to disk - PR suggestion

[oz123]

First, thank you for creating this extension and publishing it. I found it very useful!
The current cache mechanism is only in memory, and it's no so straight how to add on disk caching.
Further, if you try adding a cache mechanism trough conf.py you will bump into pickle errors.
Hence, I forked this extension and created a JSONCache class which saved the commands
and output to disk.
It's useful in cases you do 'make clean' but don't want your commands
to run again for a couple of possible reasons:

• long execution time - more commands means longer build process
• expensive execution time, for example: your command create cloud infrastructure ...

My code works, but it still lacks tests. However, if you are interested in it, I will open a PR and
we can discuss what is the best approach to create effective tests.