Clarify that cores report all keys, whether or not supported

[cxw42]

Context: https://github.com/editorconfig/editorconfig-core-test/issues/50 asked why a core had to report keys like key=value. I looked at the spec, and realized that wasn't expressly called out.

Change:

• Terminology: reference key-value pairs
• Supported Pairs: state that cores must report all key-value pairs.
• Bump version to 0.15.1

[xuhdev]

Thank you, this sounds better +1

I'm still not entirely sure why we need to support arbitrary key-value pairs but it is like it is. :)

The reasons are:

• We allow some editors to put in extra non-standard key-value pairs.
• For any further development of new standard key-value pairs, a lot of times only plugins need to be updated. In this way we avoid the need to tie core libraries to the change in standard.

[cxw42]

I realized we had both "Terminology" and "Terms" :) so also renamed the latter.

   confusion how to parse values containing those characters. Old EditorConfig
   parsers may still allow inline comments.

-Terms
------
+Parts of an EditorConfig file
+-----------------------------

-EditorConfig defines the following terms for parts of an EditorConfig file:
+The parts of an EditorConfig file are:

 - Preamble: the lines that precedes the first section. The preamble is optional