Replace defines for options with in situ values

The values are now assigned at compiled time. Hence it is not necessary
anymore to check for valid command line values at runtime and assign the
default value if necessary. Now it is sufficient to assign a new value
only if there actually is one.

This is also some preliminary work for other sources of configuration
values.
Struct for reading config values from X resources

We need to know the type of the value to process it properly on parsing.
Passing the values as addresses is necessary because num and str need to
be compile time constants for assigning them in the array.
Read configuration values from X resources

Configuration values can now be specified as X resources (e.g.
~/.Xdefaults, xrdb -load ~/.Xdefaults, man 1 xrdb).
Name or Instance is 'simpleswitcher', Class is 'Simpleswitcher'.

Possible values are:

	key
	dkey
	width
	lines
	font
	foreground
	background
	alternatebg
	highlightfg
	highlightbg
	bordercolor
	release
	startindex
	borderwidth
	opacity

E.g.:
	Simpleswitcher.key: F12
	Simpleswitcher.width: 50
	etc.
Documentation for X resources feature + resource names
Bugfix: Segfault when XResourceManagerString is NULL

Using the XResourceManagerString unchecked caused a segfault. This
happens when Xresources are not loaded, e.g. using xrdb.
Merge pull request #6 from jrk-/xresources

Xresources!