Add functionality to raise error if block or include_block is not found in file

[geel9]

The current behavior for block and include_block is to return the full contents of the file if the block is not found. This is not optimal for our use case -- a block not being found is, to us, an error which should prevent the building of our docs site entirely.

This PR:

• Adds a config option, block_throw, which defaults to false.
  • If false, behavior remains unchanged.
  • If true, the select() method will raise a ValueError if block or inside_block are not None, and the requested block is not found in the file.
• Updates README.md
  • Explains behavior if block not found
  • Adds Configuration section detailing config options
• Adds tests for new behavior

[fundthmcalculus]

@rnorth any thoughts on this ticket?

[rnorth]

I think this looks sensible - thanks @geel9 and sorry for taking a long time to get to this