Add write rest endpoint and wp cli and gui support

[bonny]

Add a write rest endpoint for writing/adding events to the log.

Example scenarios:

• Authed users can add events to the event log
• Third party plugins can write events to the log
• Third party services can add things to the log
• The Simple History plugin can add a function where admins can add events/messages to the log

Även använda samma för att logga saker manuellt? Eller en egen logger för det iaf, pga söka och hålla koll på saker. Som går att skriva till via API.

Also add wp-cli command to add event:

• wp simple-history event add
• wp simple-history event add 'Backup finished --loglevel=debug --initiator=user

See also: #110