Allow empty values in the spec

[cxw42]

Note: This PR builds on #46. Please review only the head commit. All other commits are part of #46.

Part of https://github.com/editorconfig/editorconfig/issues/496 .

Update the spec to expressly permit the form

key =

as defining key with an empty string as its value. This is different from not defining key at all.

@ppalaga has written draft tests at https://github.com/editorconfig/editorconfig-core-test/pull/52 .