Allow inline comments to disable particular warnings

[japhib]

While it's great to have a .gradient_ignore.exs file where you can specify particular warnings to ignore, it would be awesome if you could do it with inline comments as well.

For instance, when using Credo you can add a comment like # credo:disable-for-next-line Name.Of.Specific.Lint.To.Disable on the line before a warning occurs.

One reason for this is that when the warning-disabling stuff is far away from the code it's affecting, it's more likely to experience drift, becoming out of sync with the code it references. For instance, you might have a warning disabled, and then change the code so the warning no longer occurs, but forget to also update .gradient_ignore.exs to remove that warning-disabling. On the other hand, if the warning-disabling stuff is right next to the code that is being changed, it's easy to remember to update it, or for someone to look at a diff and see that the comment adjacent to the changes might not

Another reason for this is scalability. I'm in the (long :P) process of adopting Gradient for a huge Elixir project at my company that contains thousands of files and hundreds of thousands of lines of code. There are going to be hundreds of warnings we'll want to temporarily disable as we work through inaccurate type hints and other issues. If all these disabled warnings go into one single file, that file will be huge and disorganized. I'd much rather offer our engineers the option of disabling a warning via a comment in the actual file it's affecting.

Hopefully this issue makes sense! I'll be out of office for about a week starting tomorrow, but when I return, I hope to take a stab at an MR for this.

[japhib]

Implemented in this PR: #163

Could I get a review? @erszcz @luk-pau-es