slbt_output_exec: color+annotate output
    
    This patch adds functionality to slbt_output_exec that colors/annotates output
    if outputting to a terminal, or annotation is explicitly enabled.
    
    Currently annotated output includes emboldening "slibtool:" and giving it a
    magenta, emboldening and coloring the step (compile, link, install) green, and
    then annotating the "-o <output>" argument.