enhance -i performace

[JMencius]

Hi @wdecoster In the last version I submitted, a significant drop of performance using -i or --input for .gz file is observed. I did some modifications to the code to enhance the --input performace, breifly:

1. Use different version of flate2 to achieve the best performance as mentioned in https://github.com/rust-lang/flate2-rs#Backends
2. Add a 512 k buf.
   The performance is shown below:

Data	File size	command	Version	Run time
DM.fastq.gz	21G	gunzip -c DM.fastq.gz | ./chopper -q 10 -l 500 > test.fastq	Old version (0.8.0)	658 s
DM.fastq.gz	21G	./chopper -i DM.fastq.gz -q 10 -l 500 > test.fastq	Old version (0.8.0)	3060 s
DM.fastq.gz	21G	./chopper -i DM.fastq.gz -q 10 -l 500 > test.fastq	Current pull request version	759 s

Which is still worse than system-level gunzip, but close.

[wdecoster]

Awesome!