Update readme and docs regarding optional plugins

[prabhu]

It is possible to speed up the npm install step for cdxgen by omitting optional plugins.

https://docs.npmjs.com/cli/v7/commands/npm-install#omit

npm install --omit=optional @cyclonedx/cdxgen

The following languages/features would then become unavailable:

• c/c++ (Would be based on manual parsing of make and lock files)
• Python (atom-based usage detection would be unavailable)
• evinse
• obom
• container sbom