Mbed-cli

**New:**
* Serial terminal feature via `mbed sterm` command (664). This also extends the `mbed compile` command and adds `--sterm` argument, which could be combined with `-f/--flash`.
* Offline mode via `--offline` argument to `mbed import`, `mbed new`, ... (651), which forces the use of the locally cached repositories and prevents connections to remote repositories. This requires caching to be enabled and also that the target repositories have been imported at least once before*
* Classification of secure and insecure repository URLs (642). This also introduces the `--insecure` argument to `mbed import`, `mbed add`, ..., to allow any repository URL to be imported.

**Fixed:**
* PIP 10 compatibility (661)
* Specify Python compatibility checks (662)

**New:**
* New command `mbed cache` to enable/disable/manage repository caching. (627). Repository caching is now enabled by default.
* Allow different configured path for mbed-os via `mbed config` (626)

**Fixed:**
* Mbed CLI will now import the Mbed 2 SDK tools only if `mbed.bld` is present (644)
* Prevent SCM errors when repository cache is used with wrong SCM tool (643)

**New:**
* Progress bar for when importing programs and adding libraries. The functionality is based on the progress reported by `Git` and `Mercurial`. (593)
* Shortened commands/aliases for various commands (591)
* Tags/branch awareness across Mbed CLI (592)
* Use mbed-cli as python module via `python mbed` and `python -m mbed` (608)

**Fixed:**
* Behavior of Mbed CLI with local (unpublished) repositories on Windows (606)
* Printing of detected targets for `mbed detect` (611)
* Call pip with the same python executable that runs mbed CLI (613)

**New:**
* New command `mbed releases` to list release tags in current program or library and sub-libraries (576)
* Native support for symlinked repositories both on Unix-like and Windows based systems (574)

**Fixed:**
* Behavior and backwards compatibility of `mbed compiled --supported`, `mbed target --supported` (572)
* Remove oauth inline params from repo urls (570)
* Added suggestion when `mbed detect` fails (579)

**Fixed:**
* Hardcoded 'python' call and use python_cmd var instead