Create minimal C receiver tool

[staticfloat]

To prove what is or is not a problem in our Julia reception stack, we should create a minimal C reproducer to stream samples off of the XTRX using the SoapySDR API and onto disk. It should support, at a minimum, the following features:

• Specification of which SoapySDR device to load
• Specification of samplerate, frequency, gains, etc...
• The ability to stream multiple channels to multiple files (e.g. -o file -c 2 should write out file_1.iq and file_2.iq, for the two RX channels)
• Instrumentation to measure things like how long it takes to acquire a buffer, how many DMA buffers are ready, etc...

I don't think we need to support multiple devices at this time. rx_sdr can be used as a starting point.

[mbaz]

Suggestions for the UI:

• Allow user to specify how many samples to record
• Allow user to specify a delay between start of sample capturing and start of actual recording

Both of these are super useful when debugging.

[sjkelly]

https://github.com/JuliaComputing/xtrx_julia/pull/98