Add support for variable scoping

I've started looking into using this package for a project I'm working on. We'll want to be able to specify branches which might be nested within our trees, eg. branch.sub_branch. I've tested this briefly with version 0.0.7 of formulate though and I see that such variables cannot be identified:

$ python -m formulate --from-numexpr 'branch.sub_branch < 4' --variables
ERROR:formulate:TODO TRACEBACK: ('branch.sub_branch < 4', 6, 'Expected end of text')
ERROR:formulate:Error parsing: branch.sub_branch < 4
ERROR:formulate:                     ▲
ERROR:formulate:                     ┃
ERROR:formulate:                     ┗━━━━━━ Error here or shortly after

Would it be possible to add support for this? PyParsing has a specific helper method which might be useful here, delimitedList. I think the easiest for the user is to return a single variable in this case branch.sub_branch in the example above. That might mean just including the . in the definition of the Word for Variables?

Reproduce the issue with python -m formulate --from-numexpr 'branch.sub_branch < 4' --variables, then trace the expression parser's variable definition and pyparsing integration. Done means dotted names such as branch.sub_branch parse successfully and are returned as one variable without breaking existing variable parsing.