benchmark

Auto-detect whether Benchmark should produce colorized output (#126)

* Auto-detect whether to produce colorized output

Rename --color_print to --benchmark_color for consistency with the other
flags (and Google Test). Old flag name is kept around for compatibility.

The --benchmark_color/--color_print flag takes a third option, "auto",
which is the new default. In this mode, we attempt to auto-detect
whether to produce colorized output. (The logic for deciding whether to
use colorized output was lifted from GTest:
<https://github.com/google/googletest/blob/master/googletest/src/gtest.cc#L2925>.)

* Update CONTRIBUTORS, AUTHORS