core: allow message quoting to be formatted differently.
Timestamps can now be formatted independently of the buffer timestamps,
The prefix (if it's a nick) can be wrapped in a custom format (e.g <%s>), hiding the prefix/suffix delimiter in the process.
A seperate option allows the prefix/suffix delimiter to be hidden in all cases.
core: don't quote time if quoting from a free-form buffer
NB: Also won't quote time from a normal buffer if epoch_time == 0, but I don't think this edge case will come up often.