Support backticks inside defaults

[jayvdb]

FOO={env:FOO:blah} is common enough that users would want to replace blah with backticks.

i.e.

FOO={env:FOO:`do_stuff`}

The problem with that is it takes away valid syntax, as previously that would result in FOO being equal to do_stuff in backticks. While that is not likely to be a common case, it was valid.

Another option is to add a sub_type and then FOO={env:FOO:{backticks:do_stuff}} can be used. That should currently be an error "Unknown sub_type backticks", or similar, so it doesnt remove valid syntax.

This can be done atm using

DEFAULTFOO=`blah`
FOO={env:FOO:{env:DEFAULTFOO}}

But then we run into #9 as DEFAULTFOO needs to be executed even if FOO was defined externally. But trying to determine that DEFAULTFOO is unneeded is hard, as the regex currently being used only returns the innermost env:DEFAULTFOO.

[jayvdb]

#15 is a simpler case, worth doing before this one.

[jayvdb]

Another option is to add a sub_type ... {backticks:...}

The ability to inject sub_types into the tox config parser is a wonderful crazy idea which needs its own tox plugin helper library, with a dedicated regression test, or it needs support in tox. Without support inside tox, currently this would need to inject a modified Replacer, and then we'd need to be sure that any other tox plugin gets the modified Replacer with the new functionality.