Implement modify data message

[claucece]

Description:
Modify function should receive the data message and modify the text in it.

For that the user must to give the MAC key for that message (in order to generate a new mac for the modified message).

Reference:
https://bugs.otr.im/lib/libotr/blob/master/README (TOOLKIT section)
https://bugs.otr.im/lib/libotr/tree/master/toolkit
https://github.com/otrv4/otrv4/blob/master/otrv4.md#forging-transcripts
https://en.wikipedia.org/wiki/Malleability_(cryptography)

[annacruz]

Re-reading de issue it's written

Recalculate the MAC tag with the revealed MAC key associated with this
message. The new tag is attached to the data message, replacing the old
value.

So this function depends of otr_remac function implementation