First shot at adding arguments

[hughjonesd]

This just tries to return arguments as a plain string. The algorithm just gets siblings and first cousins of the original function call, excluding those which come before it. I have no deep understanding of getParseData, so buyer beware... it also doesn't deal with getParseData not returning text in its data frame, but NB that getParseText appears broken.

[codecov-io]

Codecov Report

Merging #24 into master will not change coverage. The diff coverage is 0%.

@@         Coverage Diff          @@
##           master   #24   +/-   ##
====================================
  Coverage       0%    0%           
====================================
  Files          11    11           
  Lines         224   233    +9     
====================================
- Misses        224   233    +9

Impacted Files	Coverage Δ
R/function-calls.R	0% <0%> (ø)	:arrow_up:

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