Local manga in zip/cbz/folder format (#648)

* add local source

* small fixes

* change Chapter to SChapter and and Manga to SManga in ChapterRecognition.
Use ChapterRecognition.parseChapterNumber() to recognize chapter numbers.

* use thread poll

* update isImage()

* add isImage() function to DiskUtil

* improve cover handling

* Support external SD cards

* use R.string.app_name as root folder name

app/src/main/java/eu/kanade/tachiyomi/source/SourceManager.kt

@@ -48,6 +48,7 @@ open class SourceManager(private val context: Context) {
     }
 
     private fun createInternalSources(): List<Source> = listOf(
+            LocalSource(context),
             Batoto(),
             Mangahere(),
             Mangafox(),