Adding plugin-based key-value store to SopelDB

[half-duplex]

I maintain a plugin that needs to persist some values between bot executions. This data is not specific to any channel or nickname, so the existing bot.db.{set,get}_{nick,channel}_value() functions aren't optimal, as they would require fabricating a channel or user, which has other implications including potential collisions. My interim solution of writing the values to the bot.config.pluginname is dirty since these values aren't things the bot owner should need to care about.

Proposed are the following methods within SopelDB:

get_plugin_value(plugin, key)
    """Retrieves the value for a given key associated with a plugin name."""

set_plugin_value(plugin, key, value)
    """Sets the value for a given key to be associated with the plugin name."""

[dgw]

We have already nick_values and channel_values tables, so logically adding a plugin_values table would make sense for this.

Does it need any columns besides plugin, key, and value? All probably type STRING.

Work on this would need to coordinate with #1446, probably by waiting to start implementation until the conversion to SQLAlchemy is done (then this only has to be implemented once, on the new architecture).

[dgw]

I neglected to mention, #1446 is merged, completing the move to SQLAlchemy.

[deathbybandaid]

I'll take a stab at this one