Make `bundler.sh` handle JSON file argument; Improve error handling/behaviour

#### Handle JSON files (shawnrice/alfred-bundler#1)

`bundler.sh` now correctly handles a JSON file if passed on in the arguments.

#### Improve error handling/error messages (shawnrice/alfred-bundler#2)

- Return appropriate exit code from `__loadAsset`.
- Pass through exit code of `__loadAsset` as script exit status in `alfred.bundler.misc.sh` (Similar changes are presumably necessary in `alfred.bundler.sh`, but I haven't made them).
- Return specific error message if the JSON file isn't found.

#### Python bundler

Add Python near-equivalent bundler wrapper

- Utilities/assets work as in bash/PHP bundlers
- Cannot handle versioned Python libraries, but installs a different set for each workflow (based on bundle ID)
- Uses Pip and its `requirements.txt` to specify dependencies
- Automatically updates installed libraries if `requirements.txt` has changed