fix: Create directory for merge destination.

Fixes istanbuljs#976

lib/commands/merge.js

@@ -1,5 +1,7 @@
 'use strict'
 const fs = require('fs')
+const path = require('path')
+const makeDir = require('make-dir')
 
 var NYC
 try {
@@ -49,6 +51,7 @@ exports.handler = function (argv) {
     console.error(`failed access input directory ${argv.inputDirectory} with error:\n\n${err.message}`)
     process.exit(1)
   }
+  makeDir.sync(path.dirname(argv.outputFile))
   const map = nyc.getCoverageMapFromAllCoverageFiles(argv.inputDirectory)
   fs.writeFileSync(argv.outputFile, JSON.stringify(map, null, 2), 'utf8')
   console.info(`coverage files in ${argv.inputDirectory} merged into ${argv.outputFile}`)