i-bem: Add ability to specify onSetMod callback called when specific value changed to another

[dfilatov]

One of possible proposal is to use ~ or ! to declare such callbacks:

bemDom.decl*('block', {
    onSetMod : {
        'm1' : {
            'v1' : function() {
            },

            '~v1' : function() {   // v1 -> *
                // code
            },

            '!v1' : function() {   // v2 -> v3
                // code
            }
        }
    }
});

In the example above ~v1 is an aliases for

onSetMod : {
    'm1' : {
        '*' : function(modName, modVal, prevModVal) {
            if(prevModVal === 'v1') {
                // code
            }
        }
    }
}

!v1 is an aliases for

onSetMod : {
    'm1' : {
        '*' : function(modName, modVal) {
            if(modVal !== 'v1') {
                // code
            }
        }
    }
}

[dfilatov]

cc @veged