plugins: initial support for a filetype plugin

plugins: initial support for a filetype plugin

A filetype plugin is a plugin that implements an eve filetype. Most
of the current filetypes could likely be implemented as such a plugin.
Such a plugin must implement Open, Close and Write, where Write
is provided the formatted JSON to be logged.

This commit also includes the plumbing for plugin loading. Example
plugin to come.

Plugins are loaded by the "plugin" section in the configuration
file:

  plugins:
    - /path/to/directory/plugins
    - /path/to/plugin_file.so

This can also be done on the command line with:

  --set plugins.0=/path/plugin_file.so