Make the -nodejs suffix configurable

[MegaGM]

Fixes #35

[SamVerschueren]

Should be added to the documentation as well in the readme.

[sindresorhus]

This PR needs a proper title.

[sindresorhus]

@MegaGM Would you be able to document it? Also, include your use-case about wanting to use to the same store for both Node.js and Electron, so it's clear to users what they would use the option for.

Would also be great if you could include a test. You could do something similar to

conf/test.js

Line 158 in d22f823

t.true(conf.path.includes('conf'));

by just checking that the .path property include your chosen suffix.

[sindresorhus]

Friendly ping :)