faidx with long opts + output filename + line length

[lindenb]

some Minor changes in faidx:

• added support for long_opt
• added option -o|--output (default:stdout)
• added option -n|--length to specify the length of the lines in the fasta output

[whitwham]

Rebased and merged via 1e23e1c

[tseemann]

@lindenb thanks for this! fadix needed some "TLC".

I am surprised that putchar() was used. I guess it's a macro/inline and uses stdio buffering so shouldn't be too inefficient, and makes the code simple.