Add `--version` arg to display the optic tool's version

[franklinfreitas]

A new command-line arg, --version, should be available to provide a convenient way for users to quickly retrieve the current version of the tool they are using. The tool should print the version number to the console and exit gracefully.

The click module provides an easy way to implement this. A custom method to return the version is required.

@click.version_option(get_version_custom_method())

Example:

$ optic --version
optic, version 1.4.3

Adding this feature will bring the tool in line with common CLI standards.