Support ADR format from adr-tools

[sazzer]

Is your feature request related to a problem? Please describe.
The adr-tools project offers a way for ADRs to be managed easily. It would be fantastic if this Gatsby theme was able to support those files directly.

Describe the solution you'd like
The ability for the adr-tools output files to be supported directly by this theme.

This would then allow, for example, new ADRs to be authored with adr new ..... and the files automatically work as-is.

[deviantintegral]

We investigated adr-tools back when we were first starting with ADRs, and decided that for us, the complexity of another tool wasn't worth it given the alternative of having a template file and copying it for new ADRs.

So, while I don't think anyone from Lullabot would likely work on this, we would at least consider a PR that added support for this as long as it was an optional tool.