Postprocessor: fallback keyword initial

[iamvdo]

Add a postprocessor for initial keyword. For example,

.e {
    font-size: initial;
}

will be converted to:

.e {
    font-size: medium;
}

initial isn't supported in IE, even in IE11.

Troublesome:

• when initial values depend on UAs (color for example, even if black is a standard de facto)
• when initial values are contextual (text-align is left or right based on direction, etc.)