Add support for explicit exit status, suppressing color output

[bww]

Report error exit status

Adds support for supplying an explicit exit status that will be used if any of the managed processes exit with non-zero status. This is useful to report the fact that a process crashed to whatever process invoked forego.

This functionality is enabled by supplying the newly-defined -s flag with the desired exit status.

Suppress color

Colored output can optionally be suppressed by providing the -n flag. This can be useful in situations where ANSI escape codes are not correctly interpreted and result in garbage being printed to stdout.

Misc

Cleaned up a couple tests that used format specifiers in the non-formatting t.Fatal method.