Rename `headers` to `variables` or something similar

[yichern]

From #6 (comment) and #6 (comment)

I'm also not very settled on the naming of headers because headers seems to suggest something like "Math Teacher Bot" not "bot" (ie. the value of a field, not the name of the field itself). Would a better way to name it be variables (so: variable value and variable name)? This is probably better implemented in a separate PR

re: headers -- what about header_names or header_keys? In any case, agreed that it is better done in a refactor PR later IMO.

Do we want to rename headers to variables?