Add support for gitlab snippets

[bcattaneo]

Solves #8

Hi all,

Besides adding a new service (using the current provided structure) I had to modify the following:

• Add a second header (SECOND_HEADER_[service]) for wget. API requests to gitlab snippets need to include a content-type header. I made it general so it can be used in the future with another service.
• Add a new visibility setting (VISIBILITY_[service]) because gitlab accepts three different values for visibility: public, private and internal; so I couldn't use gist's PUBLIC_[service].
• Modify help message a bit, so the user knows about gitlab's visibility, auth header (which is different than github's) and also server URL.

Please let me know what you think about the changes and phrasing.

[Cogitri]

Nice, seems to work for me: https://gitlab.com/snippets/1993633

[Cogitri]

Thank you!