Add --selectedTables switch

[acaloiaro]

ERD generation in CI/CD is a pretty common workflow, and some projects understandably will not want to introduce a memerd configuration file to their projects simply to specify which tables to include in the diagram.

This simple modification exposes the --selectedTables switch, so it's not necessary to create a configuration file to select tables for diagram inclusion.

[acaloiaro]

@KarnerTh Since there is no CONTRIBUTING.md, I wasn't sure what your process is for proposing changes, so I simply opened this PR up for you to critique. If you do wish to include this, I'm happy to do the extra work of making sure that the changes are included in the requisite documentation.

Cheers

[KarnerTh]

@acaloiaro thanks for the PR - the changes look good to me!
Please add the flag also to the readme section "Parameter and flags" (https://github.com/KarnerTh/mermerd#parametersflags) - after that I will happily merge back your changes

And thanks for pointing out the missing contribution docs, I will add them in the near future 👍

[KarnerTh]

In order to make the flag available in the next release, I will merge back that PR as it is and add the documentation afterwards

[acaloiaro]

Hey @KarnerTh -- sorry about being negligent here. I forgot to follow up on those docs.