Support for specify log prefix for junit formatter

Introducing a way to specify log prefix for JUnit formatter.

Example code:

```ruby
# features/support/junit.rb
require 'cucumber/formatter/junit'
Cucumber::Formatter::Junit.log_prefix = "FEATURE"
```

This implementation supports empty string what means formatter will not prepend any prefix for generated log file names.
If developer does not touch the prefix, it wouldn't change the current behavior, formatter prepend "TEST" for every filename