Implement functions for endpoint: Tags

[sboysel]

https://research.stlouisfed.org/docs/api/fred/#Tags

The following endpoints need to be implemented properly and consistently in fredr:

• fredr_tags() - Get all tags, search for tags, or get tags by name. (Feature/function parameters (repo branch) #28)
• fredr_related_tags() - Get the related tags for one or more tags.
• fredr_tags_series() - Get the series matching tags.

The functions are to be implemented with explicit arguments for each parameter in the FRED API (see documentation for each endpoint and the discussion in #20).

[DavisVaughan]

I'll take this one

[DavisVaughan]

Important note. The JSON example uses monetary+aggregates;weekly as the example. httr::GET transforms the + into %2B which the FRED API apparently does not accept. An alternative is to just use a space, monetary aggregates;weekly where the space turns into %20 which seems to be fine. I'm going to have a validation function that checks this.

https://research.stlouisfed.org/docs/api/fred/related_tags.html#tag_names

They actually even say:
"The '+' in 'monetary+aggregates;weekly' is an URL encoded space character."
so i feel okay with this