Add ability to specify the directory that photos/videos are stored after being taken.

See issue https://github.com/levibostian/Shutter-Android/issues/8#issuecomment-376740206 for the discussion behind this feature.

In the builder to take a photo or record a video, add the ability to specify the name of a directory for the photos/videos.

Should be pretty easy. Here is the code where the directory is hard coded currently. Here is where we give the user the ability to specify the filename so we can use that same idea to give the user the ability to specify a directory.

Tasklist:

• Give user ability to specify the directory for taking a photo and recording a video.
• Update readme docs showing that you can now specify a directory to save photos/videos. Also, update the readme docs about the manifest permissions where you can use "Pictures" by default, but you can also specify more for custom directories.

Start in lib/src/main/java/com/levibostian/shutter_android/builder/ShutterTakePhotoBuilder.kt at the hard-coded directory and filename configuration points. Trace the corresponding photo and video builders, then update the README sections for save directories and manifest permissions. Done means both capture paths accept a directory and the README documents custom directories and required permissions.