Allow modules options without disabling autoModules

[developit]

This adds an explicit autoModules:true option that forces isModuleFile() behavior even if options.modules is truthy. This is important because options.modules can be necessary to configure the behavior of postcss-modules, but currently doing so also forces postcss-modules to be run on 100% of files.

Example:

postcss({
  autoModules: true,
  modules: {
    generateScopedName: '_[hash:base64:5]'
  }
})

Current Behavior:

The above object value for options.modules is truthy, which causes postcss-modules to be executed against all files.

Behavior with this change:

Since autoModules is explicitly set to true, options.modules being a (truthy) object does not override automatic module behavior, so postcss-modules is only executed against files ending in .module.css.

[codecov]

Codecov Report

Merging #291 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #291   +/-   ##
=======================================
  Coverage   92.47%   92.47%           
=======================================
  Files          10       10           
  Lines         319      319           
  Branches      112      112           
=======================================
  Hits          295      295           
  Misses         23       23           
  Partials        1        1           

Impacted Files	Coverage Δ
src/postcss-loader.js	91.91% <100.00%> (ø)

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[egoist]

Fixed by #292