Skip render if output is unchanged

[kj800x]

Similar to #31, but this is an even simpler case.

If the function logUpdate is called multiple times with the exact same input, the second call can return early and skip the re-rendering task.

In our experience, adding this short-circuiting logic in our render loop reduced the CPU usage of the iTerm process from 11-15% to 0.1% when output is unchanging.

[sindresorhus]

Good idea 👍