Add support for ending statements on a newline

[nilium]

This adds a bitflag to the parser to enable parsing newlines as
statement sentinels in place of semicolons. Semicolons continue to
work.

This still needs work to support a backslash before EOL as a means of
ending a statement, but since there's no lookahead on tokens right now,
that's a little tricky. Might be doable by checking if the last element
of the current statement is a '\' word.

Also, as-is, this requires that opening braces be on the same line as
the section they open. I don't see any reason to change this, so it'll
probably stick even if/when I add backslash line continuations.

[codecov]

Codecov Report

Merging #20 into master will decrease coverage by 0.06%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
- Coverage   90.43%   90.36%   -0.07%     
==========================================
  Files           4        4              
  Lines        1254     1277      +23     
==========================================
+ Hits         1134     1154      +20     
- Misses        106      108       +2     
- Partials       14       15       +1

Impacted Files	Coverage Δ
parser.go	91.7% <90%> (-0.51%)	⬇️

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