Allow control of estimate and MOE suffixes in get_acs() wide format

[leehach]

When using get_acs() with data in long format, the variable columns are named "estimate" and "moe". I like this because I prefer lower case column names.

When downloading in wide format, the variables get the suffix "E" and "M". I can of course rename these columns using rename(), but it would save a step that I pretty much have to do every time I use get_acs() if I could control the suffix names in the call.

For example:

  estimate_suffix = "est",
  moe_suffix = "moe",

The defaults could remain "E" and "M" so as to not break existing code that relies upon those.

Alternatively, the suffixes could be provided by a single parameter:

  suffix = c("est", "moe"),

This parameter could be ignored when requesting data in long format, or alternatively, could be used to control the column names returned in long format. In that case, "suffix" would no longer be an appropriate parameter name, but we could some up with something reasonable.