lint-file: Add flags to operate correctly outside of projects

[carmenbianca]

#1055 adds the reuse lint-file command, but it only works inside of the context of a Project as defined by the REUSE Specification. This means:

• You can't lint files ignored by REUSE/VCS.
• REUSE.toml is checked for information.
• .license files are checked for information.
• Used licenses must appear in the project's LICENSES/ directory (linter checks for missing licenses).

Any of these may be a dealbreaker for using lint-file in scripts, particularly the missing licenses check and the inability to lint ignored files. However, because the intended use-case of lint-file is pre-commit, this is not a dealbreaker for the intended audience. We can release without this.

In order to fix this issue, the following things should be added:

• Flags for reuse lint-file to disable each of the above behaviours separately.
• A --no-project (or some such) flag that disables several/all behaviours. Certainly the ignored files and missing licences check. For .license and REUSE.toml, I'm not certain.