Allow loading effects files with '@' directive; update README.md and usage.

It is now possible to construct the effects chain using files. The syntax is
@[~/]path. The '~/' prefix will be expanded to the contents of $HOME. On the
command line, the path is relative to $PWD. Within an effects file, the path
is relative to the directory containing said effects file. If the path starts
with a '/', the path is absolute.

The effects file syntax is very simple.
  * Arguments are delimited using whitespace.
  * Comments are specified using a '#' character at the beginning of a
    line (excluding any preceding spaces or tabs).
  * The '\' character removes any special meaning of the character that
    follows it.
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