Also support syncing the file automatically.

Output to stdout (if config LOCAL_SPEC_PATH is None):

$ flask spec

Output to a file (default to using config LOCAL_SPEC_PATH):

$ flask spec -o openapi.json

Other names for the option:

• -o/--output
• -o/--output-path
• -p/--path
• -f/--file

Other names for LOCAL_SPEC_PATH:

• LOCAL_SPEC
• LOCAL_SPEC_FILE