cli: implement `create` functionality

[audrium]

Usage: reana-client create [OPTIONS]

  Create a new workflow.

  The ``create`` command allows to create a new workflow from reana.yaml
  specifications file. The file is expected to be located in the current
  working directory, or supplied via command-line -f option, see examples
  below.

  Examples:

       $ reana-client create

       $ reana-client create -w myanalysis

       $ reana-client create -w myanalysis -f myreana.yaml

Options:
  -f, --file PATH                 REANA specification file describing the
                                  workflow to execute. [default=reana.yaml]
  -n, -w, --name, --workflow TEXT
                                  Optional name of the workflow. [default is
                                  "workflow"]
  --skip-validation               If set, specifications file is not validated
                                  before submitting it's contents to REANA
                                  server.
  -t, --access-token TEXT         Access token of the current user.
  --help                          Show this message and exit.

Note that it requires loading and validating the specification file. It could be tackled later once we make the clients thin and refactor the spec upload logic to send the whole yaml file to the server side and process it there.