Added doctype-first

Closes #7. Only checks that !DOCTYPE is the first element. Doesn’t
check that it isn’t repeated. I figure that’s a job for a different
rule, but it’s open to discussion.
Added files for attr-no-duplication

DOES NOT WORK. Committing so I can easily see the diff later.
Added attr_no_duplication rule

Closes #10 and #45. Didn’t need to change the parser at all - we just
hook to the 'onattribute' function. Requires making our own
'attributes' property to maintain a list of the attributes for an
element, then a 'dupes' property to build an array of just duplicates.
Reworked Spec-char-escape regexes
Updated attribs to be objects with indices and lineCols
Remove attribsArr from DOM

Not needed past the in-order indices, but maybe in the future.