add since and until options in the CLI

[yanndegat]

1- the 2 websocket query args begin & end make ldp-tail go in a never ending loop 2- it would be great if we could specify these args in a human readable format (eg: since 2 days ago, until "3 hours ago") with the cli

ldp-tail --address "wss://gra1.logs.ovh.com/tail/?tk=demo&begin=1553601030&end=1553611040"

cc @pdepaepe