Added --indv-freq-burden as an experimental function.

git-svn-id: https://svn.code.sf.net/p/vcftools/code@954 19d466ce-640d-45be-954f-fad4d9839663
Fix to --indv-freq-burden.

git-svn-id: https://svn.code.sf.net/p/vcftools/code@955 19d466ce-640d-45be-954f-fad4d9839663
Edits to vcftools man page and website documentation

git-svn-id: https://svn.code.sf.net/p/vcftools/code@956 19d466ce-640d-45be-954f-fad4d9839663
Merge remote-tracking branch 'upstream'