BS Core utility hooks refactor

[deirdre-k]

• Review and merge this feature
• Duplicate and PR feature in vision

[deirdre-k]

This issue has evolved into a small refactor, moving code into brainscore-core and separating into two parts:

• generic plugin management/general utility hooks
• rst generation code specific to readthedocs content generation

Will then write scripts to apply core code in both language and vision

[deirdre-k]

Code to apply Core functionality is PRed in vision (PR #553 and language (PR #232). Both are waiting on Core functionality.