Allow conversion of OTRv4 decrypted plaintext into an specific format

[claucece]

See:

otrv4/pidgin-otrng#103

For this, this will help:

There is a callback that is made immediately before a message is encrypted and immediately after a message is decrypted. This callback can tweak the plaintext message as needed. For example, this could allow an application to convert formatting on a message if this would normally be done on the plaintext by some other entity while the message is in transit.