Create support to map license "keys" to names and URLs.

[digulla]

Add an API and config to map several license "keys" (i.e. apache, apache 2.0 and apache 2, apl2) to a single canonical name (example: Apache License v2).

For the HTML report (see issue #34), allow to specify the URL of the license.

Proposed config format:

[License Mapping]
Apache License v2: https://opensource.org/licenses/Apache-2.0
    apache  # One key per line
    apache 2.0
    apache 2
BSD: ...
GNU Public License v3: ...
ISC License: https://opensource.org/licenses/ISC
    isc license

[License Overrides]
# See http://zesty.ca/python/uuid.README.txt
uuid 1.3.0: isc license

If the first "line" of the mapping value starts with http:// or https://, it's a URL, otherwise a license key.