read input VCF in chunks

[seru71]

enhancement of #14

[seru71]

Currently the VCF file is read in one piece. Tested size limitations were following:

• plain text VCF in Chrome 68 can be upto 1GB,
• plain text VCF limit in Firefox 64.0 was ~250MB

FileReader API supports chunked reads, investigate if this could lift the limits.