for time_filter() specify window size

[t-emery]

Hi, first of all huge fan of all of the tidyquant related packages. They are amazing, and thank you for all of the hard work.

I was doing some analysis yesterday, and thought it might be a perfect place to use time_filter(). My most common time filtering task is to find the last x days/weeks/months/years of data, and then perform analysis on them. Looking through the tibbletime documentation, I didn't see anything for this kind of filtering, so I just used a regular dplyr filter. I think it would be a logical and awesome addition to the function, as I suspect it is a quite common use case for other people as well.

I'm quite new to GitHub, so frankly I'm not sure if the "issues" section is the right forum for functionality suggestions.

Thanks!!! You folks are great.