[debug] Allow turning on debug flags from plugins

Fix #91 (closed)

We need to delay the setting of (some) flags after the loading of plugins, hence the call to Debug.init_why3_debug in Wenv.init. But we also want to activate some flags earlier, as the whyconf flag which allow debugging the config loading (which happens before plugins loading). The chosen solution is to set the debug flags in two stages : a first try where we store the flags that failed to load in a list reference, a second try in init_why3_debug where those flags are retried and an error is raised if they definitely don't exist