Handle transaction size limit properly

One can try to send more than 100 outputs through Silkaj. For now, no warning is displayed before generating the tx document through duniterpy's Transaction() class.

We should :

• either find a max number of outputs for which we are sure that we don't reach the max size of transaction doc
• or find an algorithm to check if the max size is reached, combining input and outputs number

btw, I don't find the max size of a transaction document in the RFC.