Add % var explained in trajectory plots for q2

[cameronmartino]

This may be a bit difficult because we would like to have the percent var. explained on the axis for the volatility plot -- while still specifying it in the command.

For example in the tutorial we have:

qiime longitudinal volatility \
   ...
    --p-default-metric PC1 \
   ...

but it would be difficult to have this as "PC1 (#%)" so default % is removed. This may be helpful to build intro the trajectory types that could be a mix between Metadata and Ordination types.