Add ability to detect SPDXIDs by reserved keyname

[mrutkows]

It is a more-and-more popular convention to use the reserved keyname for declaring SPDX license IDs i.e., "SPDX-License-Identifier:" typically placed at the top of source files. The scanner must be able to look for these key-values explicitly (within the first X lines) and use the keyword to distinguish such SPDX IDs like "MIT" from words that contain "mit" in some normalized part of their text.