Add scope for includegraphics file paths

[Aerijo]

As per AtomLinter/linter-spell-latex#21 the file path should be scoped to allow the spell check packages to blacklist it. This could also be done for \biboptions and other common constructions which don't take real words.

[yudai-nkt]

Seems like AtomLinter/linter-spell-latex#20 is also related. Maybe we can scope file paths and citation keys to one of these or create dedicated scopes with a PR to linter-spell-latex.