Made a nim version of fasq aware demultiplexer.

Time comparisons (in s) on iCLIP_CSR1_090118 data:

       |   50k |      1M |
-------+-------+---------+
Python | 5.860 | 111.166 |
Nim    | 0.235 |   3.484 |
-------+-------+---------+

The Nim version doesn't handle compressed data yet.

Its call syntax is slightly different from the call syntax of the Python
version, due to the use of docopt: barcodes have to be passed one by one
with several -b options.