Make changes to the ABNF grammar for non-chainable directives

[joker314]

Resolves #150

This pull request adds:

• Allow comments and fields in between a 'Canonical' field and a 'Preferred-Languages' field
• Allow a 'Canonical' field to appear after a 'Preferred-Languages' field (i.e,. make the order irrelevant)
• Allow a 'Canonical' field or 'Preferred-Languages' field to not appear (i.e., make them optional)
• Add a history entry

Questions:

• If a Canonical or Preferred-Languages field is ommitted, the grammar will effectively become *line *line. This, I think, will lead to a parsing ambiguity (i.e., which *line gobbles up more of the lines?), and is that bad (and is it avoidable)?
• If the file is signed, I believe 'Canonical' becomes a 'SHOULD'. Do we need to make it a requirement in the grammar or is that only for 'MUST's?
• It appears when I ran make txt, indentation for the ABNF section was lost. This is probably caused by a different version of the relevant tools, should I manually go in and re-indent all of the ABNF?

[joker314]

I noticed that in the ABNF for ABNF, two spaces are placed after the equals sign so that everything lines up. However, for the URI ABNF (https://tools.ietf.org/html/rfc3986#appendix-A), instead of using =/, just / is used, and only one space is inserted after.

Which option should we use in order to keep things aligned?

[nightwatchcyber]

I think two spaces should work

The other change is good "=/"