Add `rspamd_config:add_map` lua function.

Add `rspamd_config:add_map` lua function.

This function is intended to create maps with lua callback that is
called on map reading. For example:

```
local function cb(input)
	print(input)
end

rspamd_config:add_map('http://example.com/test.map', 'cool map`, cb)
```