Enable to define the class name in the test case

I had a situation where I had a C ++ source file with several test cases and I wanted to be able to define the class name of some of them when using JUnit output. There is the `--filenames-as-tags` option, but I wanted a different name from the file name. Also, I didn't want to split my test cases into multiple files just to have different class names. Would be great to be able to set the class name in this situations.

To solve this problem quickly I introduced a new special tag prefixed with `:` (colon) that works in the same way as tags prefixed with `#`. The difference is that the user can define this new tag. I could just remove the restriction on `#`, but I chose not to change that behavior. I can make a push request with this change if people find it useful.