Support for all std mustache features

[Lucas-C]

Request from N. Boury :

it woud be great that hesperides applies mustache notations as described here https://mustache.github.io/mustache.5.html such as : last item condition non false value ({{#person?}})

cf. also https://github.com/voyages-sncf-technologies/hesperides-gui/issues/54

Note: penser à compléter la doc utilisateur templates.html

[Lucas-C]

cf. also https://github.com/voyages-sncf-technologies/hesperides/issues/62

[Lucas-C]

Demandé à nouveau par Remi D. aujourd'hui pour avoir un {{if}} en template