Add basic .xrdb/.Xresources import plugin

Adds basic support for importing .Xresources and .xrdb files as Theme
resources.

Examples of terminal color schemes in .xrdb format can be found here:
https://github.com/mbadolato/iTerm2-Color-Schemes/tree/master/xrdb

Examples of terminal themes in .Xresources format can be found here:
https://github.com/mbadolato/iTerm2-Color-Schemes/tree/master/Xresources

https://terminal.sexy also supports exporting themes is Xresources
format.

test/import_plugins/xrdb/Test3.xresources

@@ -0,0 +1,36 @@
+! special
+*.foreground:   #eddfe4
+*.background:   #000000
+*.cursorColor:  #eddfe4
+
+! black
+*.color0:       #000000
+*.color8:       #6da3b8
+
+! red
+*.color1:       #1b0c13
+*.color9:       #ca956c
+
+! green
+*.color2:       #351b27
+*.color10:      #7ca7b9
+
+! yellow
+*.color3:       #563042
+*.color11:      #a2c9bc
+
+! blue
+*.color4:       #814b64
+*.color12:      #9aad72
+
+! magenta
+*.color5:       #b56b8d
+*.color13:      #82d0b7
+
+! cyan
+*.color6:       #d0a2b6
+*.color14:      #ae96d0
+
+! white
+*.color7:       #eddfe4
+*.color15:      #e6bfcf