fix: coverage reportDir should respect --cwd (#764)

when invoked with `nyc --cwd sub-directory`, coverage reports were saved in `./coverage`, shouldn't it be `./sub-directory/coverage`?

index.js

@@ -444,7 +444,7 @@ NYC.prototype.report = function () {
   var tree
   var map = this._getCoverageMapFromAllCoverageFiles()
   var context = libReport.createContext({
-    dir: this._reportDir,
+    dir: this.reportDirectory(),
     watermarks: this.config.watermarks
   })
 
@@ -537,6 +537,10 @@ NYC.prototype.tempDirectory = function () {
   return path.resolve(this.cwd, this._tempDirectory)
 }
 
+NYC.prototype.reportDirectory = function () {
+  return path.resolve(this.cwd, this._reportDir)
+}
+
 NYC.prototype.processInfoDirectory = function () {
   return path.resolve(this.tempDirectory(), 'processinfo')
 }