Take into the forming of double bonds in Polymer builder

[daico007]

Describe the behavior you would like added to mBuild
Currently, the polymer builder doesn't take into account the type of bond forming between monomer. This is fine for when we are forming single bond. However, when forming double bond, we may need to take into consideration about the cis-trans isomerism and make sure the involved particles are planar.

Describe the solution you'd like
Give an option to create cis or trans isomers polymer. Bonus if we can support a mix of cis and trans (given config or random)