DEV: flags/show-sess-to-flags: add support for color output

DEV: flags/show-sess-to-flags: add support for color output

Highlighting a few fields helps spot them, but only if there are not too
many. What is done here is the following:
  - the first line of each stream is highlighted in white (helps find
    beginning/end in long dumps
  - fields in the form name=value where value starts with upper case
    letters are considered as a state dump (e.g. stconn state) and are
    also highlighted. This results in ~20 pairs. In this case the name
    and value use two different colors (cyan vs yellow) to further help
    find what is being looked for

This is only done when the output is a terminal or when --color=always
is passed. It's also possible to disable it with --color=never or
--no-color.