Add data wrangling functionality

[ryantibs]

• We should add data wrangling functionality as in the covidcast package for pivoting wider and longer.
• At least, we should add functionality for rbind-ing multiple epi_signal objects together, to yield a new object of class epi_signal_long.
• In the latter, remember to make a column of the epi_signal_long object called name (which we can populate from the metadata of the original epi_signal objects/).

[ryantibs]

I'm not longer really in favor of this idea. We had lots of discussion of this on #10. And as explained in my PR in #14, I think it makes sense just to go with the most basic format possible: geo_value, time_value, everything else; and not overthink it at the moment.