Parameterize decision on how to render indexed vectors

[bmaclach]

Based on discussion in the comments of #1222:

Currently when we index a vector in Drasil, the symbol retains the vector typeface. An alternative display option would be to render the indexed version of the vector without the vector typeface. For example, for vector P at index i, we currently render it as \bf{P}_i but another valid notation is P_i. This display decision should be parameterized in Drasil.

[smiths]

Sounds good @bmaclach. We won't make this a priority right now, so I've labelled it as an enhancement. When the time comes, @JacquesCarette will also need to be assigned to this issue.