[llvm-objdump] Enable disassembly color highlighting

Enable color highlighting of disassembly in llvm-objdump. This patch
introduces a new flag --disassembler-color=<mode> that enables or
disables highlighting disassembly with ANSI escape codes. The default
mode is to enable color highlighting if outputting to a color-enabled
terminal.

Differential revision: https://reviews.llvm.org/D159224