Added Additional Stave Connectors

This pull request adds the following `StaveConnector`s:
- Left Bold Double Line
- Right Bold Double Line
- Right Single Line

I added an alias for `StaveConnector.type.SINGLE` called `StaveConnector.type.SINGLE_LEFT` since it makes sense to have an opposite for the new  `StaveConnector.type.SINGLE_RIGHT` but to not break things.

Tests are included and show how the bold double lines can be used in conjunction with the `REPEAT_END`, `REPEAT_BEGIN` and `END` barline types, as well as how it looks when multiple barline and connector objects overlap.