Results 2 issues of quyi

May I ask if it is possible to add options for math superscript and subscript commands, such as `math-subscript-command` and `math-superscript-command`, whose defaults are `\sb` and `\sp`, respectively, so that...

[issues74](https://github.com/jbezos/titlesec/issues/74) mentions that option `leftlabels` generates an error, so having the `\contentslabel` command created first and then redefined can solve this problem.