Added possibility to dump backtrace

[norberttech]

Debugging php-matcher is not an easy task, especially when working with complex values/patterns.
Backtrace should simplify this process, below example of backtrace for simple match:

$matcher->match('100', '@string@')
echo (string) $matcher->backtrace();

#1 Matcher Coduo\PHPMatcher\Matcher matching value "100" with "@string@" pattern
#2 Matcher Coduo\PHPMatcher\Matcher\ChainMatcher (all) matching value "100" with "@string@" pattern
#3 Matcher Coduo\PHPMatcher\Matcher\ChainMatcher (scalars) can match pattern "@string@"
#4 Matcher Coduo\PHPMatcher\Matcher\ChainMatcher (scalars) matching value "100" with "@string@" pattern
#5 Matcher Coduo\PHPMatcher\Matcher\CallbackMatcher can't match pattern "@string@"
#6 Matcher Coduo\PHPMatcher\Matcher\ExpressionMatcher can't match pattern "@string@"
#7 Matcher Coduo\PHPMatcher\Matcher\NullMatcher can't match pattern "@string@"
#8 Matcher Coduo\PHPMatcher\Matcher\StringMatcher can match pattern "@string@"
#9 Matcher Coduo\PHPMatcher\Matcher\StringMatcher matching value "100" with "@string@" pattern
#10 Matcher Coduo\PHPMatcher\Matcher\StringMatcher successfully matched value "100" with "@string@" pattern
#11 Matcher Coduo\PHPMatcher\Matcher\ChainMatcher (scalars) successfully matched value "100" with "@string@" pattern
#12 Matcher Coduo\PHPMatcher\Matcher\ChainMatcher (all) successfully matched value "100" with "@string@" pattern
#13 Matcher Coduo\PHPMatcher\Matcher successfully matched value "100" with "@string@" pattern

More complex matchers will generate huge backtraces but when going through them step by step it's easy to track all mistakes.