allow custom heading options

[i-just]

Description

Follow up to #434 and #459.

434 fixed a bug where the “Heading Levels” setting wasn’t fully respected and could be overwritten via “Config Options”.
459 allowed for custom heading elements to be provided via “Config Options”

This PR fixes a bug where:

• if you selected h1 and h2 under “Heading Levels” and then provided a custom config for them via “Config Options” (e.g. changed the title or added classes), that custom config wasn’t respected
• if you selected h1 and h2 under “Heading Levels” and then provided a custom config for only one of them via “Config Options”, the other heading wasn’t showing as a dropdown option

Related issues

#461

[jamiepittock]

@i-just can you please bump this a review and merge please?

[jrrdnx]

Any update on this getting merged? This is currently a roadblock for several time-sensitive projects for us.