Cleanup and stabilize Beater-API

[urso]

Simplify common API for creating a new beat:

• FlagHandler registry instead of beat optionally implementing HandleFlags. FlagHandler will be called after parsing flags, but before reading any config files. Replace optional FlagsHandler interface with callback list #2002
• simplify Beater interface to consist of: Constructor(Initialization + Config), Run and Stop. Simplify Beater interface #2003
• turn publisher.Publisher into interface Turn libbeat.Publisher into interface #2009
• pass beat specific config section to beat on init Update Beat launcher to pass beater config section to New #2014
• update newbeat docs