Support soxr custom recipes.

MPD uses soxr with prefined resample recipes. Soxr also support defining a recipe your self.
This commit will support a custom recipe by changing the existing quality setting to "custom".

The same structs as the predefined recipes uses can now set by hand.

This will make the following settings available:
- precision 16|20|24|28|32 bits, example "28"
- phase_response - 0-100, example "45"
- passband_end - used bandwidth of source 80-99.7%, example "99.7.0"
- stopband_begin - anti aliasing 100.0+%, example "100".
- attenuation - signal reduciton in dB's, 0-30. example "3.0".
- flags "0" - additional bitmask with extra settings

The data is set in the structs soxr_quality_spec and soxr_io_spec (found in soxr.h).

doc/plugins.rst

@@ -746,6 +746,25 @@ Valid quality values for libsoxr:
 * "medium"
 * "low"
 * "quick"
+* "custom"
+
+If the quality is set to custom also the following settings are available:
+
+   * - Name
+     - Description
+   * - **precision**
+     - The precision in bits. Valid values 16,20,24,28 and 32  bits.
+   * - **phase_response**
+     - Between the 0-100, Where 0=MINIMUM_PHASE and 50=LINEAR_PHASE.
+   * - **passband_end**
+     - The % of source bandwidth where to start filtering. Typical between the 90-99.7.
+   * - **stopband_begin**
+     - The % of the source bandwidth Where the anti aliasing filter start. Value 100+.
+   * - **attenuation**
+     - Reduction in dB's to prevent clipping from the resampling process.
+   * - **flags**
+     - Bitmask with additional option see soxr documentation for specific flags.
+
 
 .. _output_plugins: