Support adding a baseline

[Daniel Imms]

Proposed CLI:

xterm-benchmark --baseline

This would let us compare 2 commits:

$ xterm-benchmark --baseline

Text render layer time: 10ms
Throughput: 100ms

(do some changes)

$ xterm-benchmark

Text render layer time: 10ms -> 15ms (in red)
Throughput: 100ms -> 90ms (in green)

[jerch]

Fixed in #3