Support reading a file from the --description flag

Support using a --description=description.txt when creating a release gitlab#199253 (comment 285844201)

• Check if the value of --description is an existing file, if it is
• Load contents of the file
• Parse and validate markdown
• consider sanitizing content https://github.com/gomarkdown/markdown#sanitize-untrusted-content
• if `--description="some text", then use as string

release job:
  stage: release
  script:
    - echo "Create a new release"
  release:
    name: "Awesome release"
    description: description.txt