Use relative path in filepath matching

[stak]

EditorConfig permits relative path in section names:

# Indentation override for all JS under lib directory
[lib/**.js]
indent_style = space
indent_size = 2

(from: http://editorconfig.org/)

But above config fails because textlint-rule-editorconfig compares absolute path of the target file with patterns written in .editorconfig.

new Minimatch('lib/**.js').match('/full/path/to/lib/some.js') // => false

This PR fixes it.

[codecov-io]

Codecov Report

Merging #3 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #3   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          15     15           
  Lines         167    173    +6     
=====================================
+ Hits          167    173    +6

Impacted Files	Coverage Δ
editorconfig/get-root.js	100% <100%> (ø)	:arrow_up:
editorconfig/get-config.js	100% <100%> (ø)	:arrow_up:

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[arrowrowe]

Awesome! :tada: Thanks w