Label insertions & deletions explicitly in VCFTable

Fixes #70.

Here's a before:

![screen shot 2014-10-23 at 1 08 11 pm](https://cloud.githubusercontent.com/assets/98301/4757808/d0652332-5ad9-11e4-9827-1711f0cd2c87.png)
After:
![screen shot 2014-10-23 at 1 25 35 pm](https://cloud.githubusercontent.com/assets/98301/4757812/d2a35556-5ad9-11e4-8cdb-8d7404ff3847.png)

The formatting is definitely open to discussion!