Fix error that occurs when a failing assertion tries to format the `_` (anything) matcher.

[stringTrimmer]

Fixes #179

Cause the issue with:

   local test = spy.new(function() end)
   test('this', 39)
   assert.spy(test).called_with('that', match._)

Implementation:

• Thought about using a custom formatter for the _ matcher, but the order that the formatter is added to the list would be critical (it would have to be added after assert:add_formatter(fmt_matcher) in formatters/init.lua so it would be evaluated before fmt_matcher). And it would be another formatter to cycle thru for all arguments formatted. Plus it is a matcher after all and there already is a fmt_matcher
• Considered naming it anything and defining it like all the regular matchers. But the regular matchers are intended to be called (e.g. assert.spy(s).was.called_with(match.is_number())) whereas match._ as shown in the README is not. Plus it looks more like natural lua when used as is.
• Ultimately decided on a small change to fmt_matcher and a couple additional fields on match._ to make pass thru fmt_matcher without error.

Formatting:

• I chose to display it in the assertion messages as _ *anything* since _ alone might confuse someone reading test results who hasn't used that matcher before.

[Tieske]

@stringTrimmer thx for the fix. And apologies for the long delay.

[stringTrimmer]

np, happy to have contributed a tiny bit to this project.