merge v1.4 into master

[bxparks]

• 1.4 (2020-12-09)
  • Add 'dict' as a third input_format when SchemaGenerator is used as a
    library. This can be useful when the data has already been transformed
    into a list of native Python dict objects (see Schema generation from a list #58, thanks to
    ZiggerZZ@).
  • Expand the pattern matchers for quoted integers and quoted floating point
    numbers to be more compatible with the patterns recognized by bq load --autodetect.
  • Add Table of Contents to READMD.md. Add usage info for the
    schema_map=existing_schema_map and the input_format='dict' parameters
    in the SchemaGenerator() constructor.