Use open-api plugin settings instead of global ones

[w666]

• This is to respect setting download and display download link for api spec
• Use the same URL as a provided to plugin when generate download URL, so it does not try to download with a page prefix

Detailed description:

In my example I have the following structure:

docs-root
    |--API
    |     |-- my-api.md - this is where I render my spec file 
    |
    |-- api-specs
           |--- my-spec.yaml -- this is actual spec

I provide relative path in MD page, for example:

openapi
./api-specs/my-spec.yaml

this page has URL example.com/doc/api/my-api/, so download link is example.com/doc/api/my-api/my-spec.yaml which is not correct, it does not match the path. Instead it should be example.com/doc/api-specs/my-spec.yaml

Fixes #210

Type of change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📝 Documentation update

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings