Handle escaped identifiers

[Jackenmen]

Fixes #27

[jpgrayson]

Thanks for this PR, @Jackenmen.

An observation: this implementation follows the spec regarding escaped identifiers being terminated by white space, but all the example escaped identifiers from Vivado contain trailing backslashes (e.g. \<const0>\). Perhaps Vivado uses trailing backslashes instead of or in addition to white space to terminate escaped identifiers? This wouldn't be the first example of someone's VCD implementation straying from the IEEE standard. Anyway, I don't see an action to take at this time.