Add support for grouping

[alvarocavalcanti]

We use Lucidchart at work and it supports a couple more tags than this plugins. One that we use a lot is the grouping ones, namely alt and opt. You can see their documentation here: https://lucidchart.zendesk.com/hc/en-us/articles/208029986-UML-Sequence-Markup

If the plugin had this support we could store our sequence diagrams on the repos themselves, and not rely on an external tool.

To be honest, I might take a stab at this later, adding the support, just have do download and have a look at the code. But I'm creating this ticket so others might jump to it if available.