FIX: function call chaining in expressions

[klauer]

Closes #93

• Expressions may include chained function calls
• Prior to this PR, only statements defined in such a way were supported
• Adds chained_function_call grammar rule and ChainedFunctionCall class

[codecov-commenter]

Codecov Report

Merging #94 (7b1a286) into master (f0783f5) will increase coverage by 0.0%. Report is 1 commits behind head on master. The diff coverage is 100.0%.

@@          Coverage Diff           @@
##           master     #94   +/-   ##
======================================
  Coverage    81.5%   81.6%           
======================================
  Files          27      27           
  Lines        5819    5829   +10     
======================================
+ Hits         4747    4757   +10     
  Misses       1072    1072           

Files	Coverage Δ
blark/tests/test_transformer.py	94.7% <ø> (ø)
blark/transform.py	98.3% <100.0%> (+<0.1%)	:arrow_up: