persistent history implementation

[slingamn]

This is a successor to #309 and relates to the datastore discussion on #343.

In order to compete with Slack and Mattermost, we should have some kind of facility for persistent, searchable logs that can be viewed through a web interface, queried with CHATHISTORY, and potentially made available for full-text search. Options:

• Support logging traffic directly into files, which can then be served by an ordinary web server
• Support storing messages in a persistent (non-buntdb) datastore

[slingamn]

Closed in #792