Don’t use chalk.stripColor

To prepare for Chalk v2

Author: sindresorhus

lib/concordance-options.js

@@ -1,6 +1,7 @@
 'use strict';
 const ansiStyles = require('ansi-styles');
 const chalk = require('chalk');
+const stripAnsi = require('strip-ansi');
 const cloneDeepWith = require('lodash.clonedeepwith');
 const reactPlugin = require('@concordance/react');
 const options = require('./globals').options;
@@ -119,7 +120,7 @@ const colorTheme = {
 
 const plainTheme = cloneDeepWith(colorTheme, value => {
 	if (typeof value === 'string') {
-		return chalk.stripColor(value);
+		return stripAnsi(value);
 	}
 });