Fix #593

Exclude sometimes doesn't work with absolute path because it tries to match the pattern directly with the path itself.

Since the exclude option is relative, the absolute path to lint should be converted to relative path as well.

exclude = ["**/file.lua"]
exclude = ["src/**"]
exclude = ["external/*", "*.spec.lua"]

For example, if we are in selene-test folder with the following exclude option:

exclude = ["src/**"]

Then, by converting this absolute path /path/to/selene-test/src/file.lua to src/file.lua, exclude can work properly.

The problem is which path we should relate to, initially I thought using config directory (where selene.toml lies) is a good place, but to keep it simple without unnecessary changes, I use the current directory (where selene is invoked) instead.