flexibility on overriding config values

[viveksahu26]

close: #125

This PR provides the flexibility to override any individual field (e.g., just "version") without being required to provide all the other grouped fields. If any value along with config file are provides, then those flag value will be prioritize over config default values.

For example:

• override version value from config value:

  $ sbomasm assemble -v "v2.2.2" -c config.yaml -o final-sbom-config.spdx.json sbomex-cdx.json sbomgr-cdx.json

• override name, type, version values from config values:

  $ sbomasm assemble -v "v1.0.20" -n "bob"  -t "library"  -c config.yaml -o final-sbom-config.spdx.json sbomex-cdx.json sbomgr-cdx.json

  To understand issue more detail, see here