Pass sort and suffix to merge.data.table

[ruaridhw]

Allow passing of sort and suffix from dplyr to merge.data.table method in underlying join functions.

Fixes #40

[codecov-io]

Codecov Report

Merging #41 into master will decrease coverage by 0.58%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
- Coverage   78.01%   77.43%   -0.59%     
==========================================
  Files           8        8              
  Lines         232      226       -6     
==========================================
- Hits          181      175       -6     
  Misses         51       51

Impacted Files	Coverage Δ
R/joins.R	100% <100%> (ø)	⬆️
R/utils.R	84.37% <0%> (-2.47%)	⬇️

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[krlmlr]

This looks obsolete after #48.