Support parsing RGBA values from theme files

[Sergey Ukolov]

Some of the Text Mate themes contain color definitions like this:

<key>lineHighlight</key>
<string>#FFFFFF0F</string>

where 0F is an alpha value for the color.
But Amp does not respect alpha value which results a broken theme appearance when, for example, current line becomes highlighted with solid white color:

I understand that transparency is not supported in terminals so I propose to read background color first and then blend all other RGBA colors over it which will result desired colors in most cases.