Report correct source position for quasiquoted parsers (see #7)

Error messages are still terrible, but at least the source position
gives a clue.  This changes the types of `parseAPI` and
`parseAPIWithChangelog`, but most users will be using the
quasiquoters and so will be unaffected.
Merge pull request #25 from iconnect/issue-7

Report correct source position for quasiquoted parsers (see #7)