Helpers: detect colors by NETTE_TESTER_COLORS env

This fixes problem with coloring in Job output on Linux. Issue is in Helpers::detectColors(). The STDOUT stream is defined only if PHP is compiled as CLI. Replacement for CGI is fopen('php://stdout', 'w') but posix_isatty() still returns FALSE, because stdout is proc_open() buffer and not a terminal.
color usage is configurable via option --color
CommandLine: added support for --option=value
added Dumper::removeColors()
added Dumper::dumpException()
Tester\Helpers::with() moved to Assert
Revert "removed Assert::throws(), it is replaced by Assert::exception() (BC break)"

This reverts commit bc78dd48d34fcb232392ec0df5a5f311791602fa.
the most of methods from Tester\Helpers moved to new class Tester\Environment