Adding configuration for channel receives as sources.

[victornicolet]

This PR adds the option to specify receives on channels of a given type to be considered sources. To specify that receiving data from a channel of type chan A where A is a type in package mypackage, add the following configuration:

sources:
    - package: "mypackage"
       type: "chan A"
       kind: "channel receive"

Other minor changes:

• slight changes in position reporting. Now the position of the call is reported for call arguments, instead of the position of the SSA value, which was hard to match to the call.
• new option to suppress warnings.