Cater for automatically detected bigrams

[bobbingwide]

In bobbingwide/bigram#4 I've added some code to automatically detect SB's in content and convert them into links.

We need to cater for situation where Genesis creates a more link which contains an SB in aria text. The pragmatic solution is to use our own more text; "more".

[bobbingwide]

We should also create a 404 page that's aware of failing links.

[bobbingwide]

The code that detects SBs in context needs a few more improvements.

1. Process SBs in Category descriptions.
2. Detect SBs starting immediately after an HTML tag
3. Cater for accented SBs
4. Avoid self linking - consider using <mark>.
5. Support local development for content being copied into Word - either produce links to the live site or just emphasis tags - so that SBs appear blue in the printed book.

[bobbingwide]

Also detect SBs in the Author box ... changing About bigram to Self Bio along the way.

[bobbingwide]

The more link needs some styling improvements. It doesn't stand out well enough as a plain link.