rename payee_mappings to payee_alias_mappings

(and similar renames) in preparation for payee_uuid_mappings
support payee uuid directive

The semantics of this are a little bit tricky: we want, if we come
across a transaction with exactly the given UUID, to set the
transaction's payee to be the specified one.

We need to set that payee before the first post in the transaction is
parsed, otherwise that post will inherit the wrong payee; however, we
need to do it after the transaction's tags have been parsed.  The
implementation as it is in this commit is potentially a little
wasteful, if there are post-like (non-comment non-assertion) entries
in the transaction that don't successfully parse as posts.
document payee uuid subdirective
Merge pull request #289 from csrhodes/payee-uuid-mapping

Payee uuid mapping