Support hash redirects / heading redirects

[reteps]

Resolves #69

• First hash is used if no top-level one found
• Injects into existing pages if needed

I won't claim this is the cleanest implementation, but it works! I tested it on my own site with these rules:

      redirect_maps:
        index.md#foo: 'concepts/index.md#bam'
        foo.md#bar: 'concepts/index.md#bam'
        foo.md: 'getting-started/index.md'

where foo.md doesn't exist, and index.md does.