Add respect for extension property in options file

[ghost]

#63 resolved. Previously, it was not possible to define the extension in an options file.

[ghost]

Failing tests are a result of pug dependencies:
Command failed:
/home/travis/build/pugjs/pug-cli/node_modules/pug/node_modules/pug-parser/index.js:391

    const expr = this.parseExpr();
    ^^^^^

SyntaxError: Use of const in strict mode.

[codecov-io]

Codecov Report

Merging #64 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #64   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines         145    146    +1     
=====================================
+ Hits          145    146    +1

Impacted Files	Coverage Δ
index.js	100% <100%> (ø)	⬆️

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[ghost]

pugjs/pug-parser#31

Issue causing failing builds

[ghost]

The const/var in strict mode issue has been resolved, but is not pushed to NPM yet:

pugjs/pug@e9dbadd

This pull request should solve these build issues.