Add new summary-output option to CRH

[tonyelewis]

Add this two options to the CRH output options block:

• summary-output Output a brief text summary of the input data (rather than processing it)

Jon and I think this option would be useful.

[tonyelewis]

Discussed with Jon: perhaps --summary-output should be --summarise to make it clearer / more concise.

[tonyelewis]

Also:

• place --summarise above --html-output
• make the crh_output_spec enforce:
  • restrict to at most one of --summarise and --html-output
  • disallow --output-trimmed-hits with any of --summarise or --html-output
  • disallow --restrict-html-within-body with --summarise

Edit: changed to allow for fact that --export-css-file takes a file argument and is independent from the output format options.