See https://github.com/pypa/warehouse/issues/6345 and https://github.com/pypa/warehouse/pull/6368

* Improve the export command

* Update documentation

Mkdir is breaking when folder is already existing, this allows it not to fail if target exists

* Add `-o` flag for `export` command

The export command now prints to standard output by default.  To store the
output in a file, the new `-o <filename>` argument can be used.

* Fix code style

* Improve usage examples for `export` command

* Add missing import statement to exporter module

* Make Exporter class work w/ IO objects for output

* Fix test suite for `Exporter` utility class

* Refactor configuration code

* Add support for local configuration

* Update documentation

* Add the ability to specify path and git dependencies directly in add

* Add the ability to add git and path dependencies via the init command

* Automatically select latest prereleases if only prereleases are available

* Update documentation for the add command

* Add the ability to add complete dependencies in one go

Co-authored-by: rusty <rusty@spoqa.com>