Add ability to apply filters to specific data source only

[bostjan]

Currently message format can be specified like this (in snoopy.ini):
message_format = "ds1=%{datasource1} ds2=%{datasource2:param1,param2}..."

Then only whole message can be filtered, like this:
filter_chain = "exclude_uid:1,2,3;exclude_spawns_of:crond"

It would be splendid if filters could be applied to only specific datasource, right in message format, like this:
message_format = 'ds1:"%{ds1|escape_json_str}" ds2="%{ds2:param1|escape_json_str}"...'

The question that needs to be answered is: do we implement this among existing filters, or do we create new entity class (modifiers, manglers, etc) which does only data mangling.

[bostjan]

Maybe @evilezh will be willing to contribute escape_json_str() code :)

evilezh@9e7aeaa

b.

[bostjan]

Seems unnecessary for the time being.