[compare.py] Add --split-by-metric option (#395)

This patch adds a new option to split the output view of `compare.py` vertically by metric, to make it more scalable as statictical columns and metric count increases. For example, comparing CTMark results for exec time, compile time, code size, and max RSS (added in https://github.com/llvm/llvm-test-suite/pull/373) gets broken on a usual laptop.

Currently the summary output is still pivoted by metric because its static and more compact.