Ignore "line" PIs

[martinthomson]

kramdown-rfc adds <?line 100?> processing instructions to the XML it generates. This appears to help xml2rfc include useful line numbers in its error reporting.

This is probably fine, except in the strictest modes, but I'm getting errors like this:

 2  Error
 └- Code - INVALID_ABSTRACT_SECTION_CHILD
 └- Desc - The abstract section must consist of <dl>, <ol>, <t> or <ul> elements only.
 └- Ref  - https://www.rfc-editor.org/rfc/rfc7991.html#section-2.1
 └- Path - rfc.front.abstract.?line