suggest merge 'nbc-change-at','bc-change-at','editoral-change-at' into one extension

[lllyfeng]

'nbc-change-at','bc-change-at','editoral-change-at' are all used to describe the change information, it's difficult for implementing 3 extensions, because the change history should be list in time order regardless the change type, but every extension should be treated a indepent statement, YANG parser don't know the relationship between these 3 extensions.

I suggest use a one extension to express this information.

extension change-at {
argument revision-date-or-label;
}

and another extension 'change-type' can be used in extension 'change-at' as the sub-statement to indicate the compatible information.
extension change-type {
argument type;
description "the change-type can be nbc,bc,editorial".
}

for example:
list sir-changed-a-lot {
rev:change-at "3.0.0" {
rev:change-type "editorial";
}
rev:change-at "2.3.0" {
rev:change-type "bc";
}
rev:change-at "1.2.1_non_compatible" {
rev:change-type "bc";
}
description "a list of stuff";
ordered-by user;
key "foo";
leaf foo {
type string;
}
leaf thing {
type uint8;
}
}