Adds benchmarking using matcha.

Run through:
```shell
./node_modules/matcha/bin/matcha benchmark.js
```

Results against current HEAD:
```shell
1,863,195 op/s » add colour
2,215,812 op/s » add several styles
  323,213 op/s » add nested styles
```

Results against the latest revision before optiziations, e122899
```shell
  26,714 op/s » add colour
  26,752 op/s » add several styles
  13,414 op/s » add nested styles
```

Closes #21
Adds performance to readme.