[feature] add support for "exportable" flag

[masukomi]

There are many little scripts we can write that only apply for the creator. Either they'll only work on the creator's system, or they depend on other tools / creations that only the creator has.

You might also be documenting some common unix tool that has bad "man pages" and want to show up in tooloo.

Regardless of why, there is stuff that you'll document that you don't want to be included in statically generated sites or other future export forms.

This task is to add support for an exportable flag in the TOML

• add to command's DB table
• add to upgrade tooling for people using current version
• add observation of the loop in the export that iterates over commands OR make the query just not include these in results.