Improve PubSec authfile formats handling, test, document

Following #273 (closed) and #278 (closed):

check compatibility with Cesium's and Duniter's PubSec format
Add PubSec compatibility to the regex in DuniterPy
Write tests for PubSec authfile formats into DuniterPy
Import DuniterPy regex for filtering the authfiles in Silkaj

#271: document accepted authfile formats

Edited Jan 25, 2020 by Moul

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