Allow base contract string constructor args with open curly braces

[cgewecke]

#478

contract Test is Interpolated("abc{defg}"){
    function a(uint x) public {
        uint y = x;
    }
}

[codecov-io]

Codecov Report

Merging #479 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #479      +/-   ##
==========================================
+ Coverage   99.22%   99.23%   +<.01%     
==========================================
  Files          20       20              
  Lines         779      784       +5     
==========================================
+ Hits          773      778       +5     
  Misses          6        6

Impacted Files	Coverage Δ
lib/parse.js	96.05% <100%> (+0.27%)	⬆️

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