Isort

- Fixed 1381: --combine-as loses noqa in different circumstances.

- Implemented 1373: support for length sort only of direct (AKA straight) imports.
- Fixed 1321: --combine-as loses noqa.
- Fixed 1375: --dont-order-by-type CLI broken.

- Fixed incorrect warning code (W503->W0503).

- Improve upgrade warnings to be less noisy and point to error codes for easy interoperability with Visual Studio Code (see: 1363).

- Implemented ability to treat all or select comments as code (issue 1357)
- Implemented ability to use different configs for different file extensions (issue 1162)
- Implemented ability to specify the types of imports (issue 1181)
- Implemented ability to dedup import headings (issue 953)
- Added experimental support for sorting literals (issue 1358)
- Added experimental support for sorting and deduping groupings of assignments.
- Improved handling of deprecated single line variables for usage with Visual Studio Code (issue 1363)
- Improved handling of mixed newline forms within same source file.
- Improved error handling for known sections.
- Improved API consistency, returning a boolean value for all modification API calls to indicate if changes were made.
- Fixed 1366: spurious errors when combining skip with --gitignore.
- Fixed 1359: --skip-gitignore does not honor ignored symlink

Internal Development:
- Initial hypothesmith powered test to help catch unexpected syntax parsing and output errors (thanks Zac-HD!)

- Fixed 1356: return status when arguments are passed in without files or a content stream.