polls_filter: Select polls
In heike/electionViz: Visualizing US Polls and Election Results

Filter polls based on k most recent by state or since a set date.

1	polls_filter(polls, method = "last_k", k = 5, since = today() - weeks(4))

`polls`	data frame of polling results as returned by 'fivethirtyeight_update“
`method`	which method do we use to include polls: "last_k" includes the last 'k' polls, "since" includes polls as specified in 'since'.
`k`	include k most recent polls
`since`	date after which polls to include (based on 'date' or 'end_date').