Add experimental LADSPA fontend (build with ./build_ladspa.sh).

Currently, this frontend will not work correctly with effects that add latency
between their input and output (currently only crossfeed_hrtf).

The default configuration file is located at
$XDG_CONFIG_HOME/ladspa_dsp/config (override by setting the LADSPA_DSP_CONFIG
environmnent variable) and is a simple key-value format. Whitespace is not
ignored. Valid keys are:
	input_channels    - number of input channels
	output_channels   - number of output channels
	effects_chain     - args to build the effects chain

Example configuration:
	# Comment
	input_channels=2
	output_channels=2
	effects_chain=gain -4.0 lowshelf 90 0.7 +4.0 @eq_file.txt