Provide the ability to maintain a custom license collection separate from the standard ScanCode licenses

[DennisClark]

A business may discover multiple commercial and other proprietary licenses in its product code, or in third-party products that it is evaluating for usage. These notices often appear as proprietary copyright and/or license notices in source file headers. Since the information in those license notices might be sensitive, it would be very useful to:

Collect these notices+detection rules in a custom directory path, which could then be used in addition to the standard ScanCode licenses by a runtime option.

Designating a custom directory path outside of the standard ScanCode directory would help to manage this custom collection and help to protect it from getting wiped out by a ScanCode upgrade.

[pombredanne]

@DennisClark Thanks!
if you think this is the same as #480 it may make sense to merge that in this ticket

[DennisClark]

Closing as duplicate of #480