A variable can contains digits (#35)

[alexander-zubkov]

Variables can contain digits. It may not be an elegant solution, but it works.

[fkleon]

Thanks for this! Happy to include a not-so-elegant change, if it improves things.

Two comments:

• Noting that this change wouldn't allow a digit at the beginning of a variable, right?
• Could you add some unit tests for various scenarios?

I've fixed the unrelated lint failure on the master branch.

[fkleon]

Thanks for your contribution. I've finally revisited this and added some unit tests in 0c3f79c