Add ability to choose a file in source_gist()

[ateucher]

This adds an argument filename to the source_gist() function so that a user can specify a particular file in a gist that has more than one (currently it will source the first .r file in a gist).

e.g.:

source_gist("gist.github.com/ateucher/0689a1460a0ac136d3b69c371de3a57e", filename = "hi.r")
source_gist("gist.github.com/ateucher/0689a1460a0ac136d3b69c371de3a57e", filename = "hello.r")

[codecov-io]

Current coverage is 41.81%

Merging #1172 into master will increase coverage by +<.01%

@@             master      #1172   diff @@
==========================================
  Files            86         86          
  Lines          4464       4475    +11   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits           1863       1871     +8   
- Misses         2601       2604     +3   
  Partials          0          0          

1. File R/github.R (not in diff) was modified. more
   • Misses -8
   • Partials 0
   • Hits +8

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[ateucher]

Awesome, thanks!