Support modification of GitLab data

[nbigaouette]

Right now API is read-only; data can be retrieved but not set.

To be useful the API should be extended to write too.

[nbigaouette]

Some fields in the different struct do make sense to modify, for example the title or content of an issue, but others are really just read-only property, for example the creation date.

The crate should provide either:

1. Methods to modify a specific field, a la builder pattern;
2. Allow the user to directly modify directly a struct that would be sent.

Option 1. might require more work, but it's probably a better option as it allow to use the type system for static validation.