Add support for virtual keyword and abstract contracts

[fvictorio]

Part of #221.

Now abstract contract contracts are handled, and so are functions with the virtual keyword.

The override part is blocked by Consensys/solidity-parser-antlr#2

[codecov]

Codecov Report

Merging #228 into master will increase coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #228      +/-   ##
==========================================
+ Coverage   99.01%   99.21%   +0.19%     
==========================================
  Files          74       78       +4     
  Lines         611      635      +24     
  Branches      101      103       +2     
==========================================
+ Hits          605      630      +25     
+ Misses          6        5       -1     

Impacted Files	Coverage Δ
src/nodes/CatchClause.js	100.00% <0.00%> (ø)
src/nodes/DoWhileStatement.js	100.00% <0.00%> (ø)
src/nodes/TypeNameExpression.js	100.00% <0.00%> (ø)
src/nodes/AssemblyFunctionDefinition.js	100.00% <0.00%> (ø)
src/nodes/TryStatement.js	100.00% <0.00%> (ø)
src/nodes/ContinueStatement.js	100.00% <0.00%> (+33.33%)	⬆️

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