feat(gatsby-transformer-remark): add options for tableOfContents

[byzanth]

Added the ability to pass options to mdast-util-toc from gatsby-transformer-remark.

closes #11452

[LekoArts]

@pieh Anything else left that needs to be done? 😊 LGTM

[gatsbot]

Holy buckets, @byzanth — we just merged your PR to Gatsby! 💪💜

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[pieh]

Thanks for your work on this @byzanth and sorry it took so long to get this merged!