Persist additional options / flags in the config on `--install`

[casperdcl]

e.g. nbstripout --install --keep-count should put the --keep-count flag in the resultant git config filter.nbstripout.clean and git config diff.ipynb.textconv commands.

This applies to:

• --keep-count
• --keep-output

[kynan]

Great suggestion :) Feel like sending a PR for this @casperdcl ?

[kynan]

Would you be interested in working on this @casperdcl ?

[GniLudio]

Until this is resolved, these commands can be used:

git config filter.nbstripout.clean "$(git config filter.nbstripout.clean) --your-flag"
git config diff.ipynb.textconv "$(git config diff.ipynb.textconv) --your-flag"