ScanCode.io: Support multiple tools adding gettext for strings

[DennisClark]

Here we add strings extraction in ScanCode.io pipeline for symbols.
We should use the source-inspector support to collect source code strings via xgettext and add these to the default pipeline for now. Let's call this an add-on pipeline collect_source_strings

Later we will want to make the default pipeline less fat:

• PurlDB: make symbols and strings indexing a separate, on demand option purldb#376

[pombredanne]

The new addon pipeline is working as designed! Try it by adding "collect_strings_gettext" to a project with an existing primary pipeline