Support custom variables on step level

At the moment environment variables for Bitbucket Pipelines are predefined in the repository and team configuration.

I'd like to propose adding support for prompting predefined environment variables from the end-user when starting a pipeline or a manual step inside a pipeline. In addition it would be really useful to be able to validate the values using a regular expression.

My use-case for this functionality is performing a bugfix release where the user is prompted for the to-be-released and next version numbers before the release is started.