docs: Targets accept "^" for logical complement
Add -o FILE / --output FILE option to redirect output
merge: Check for tags of conflicting lengths
Merge pull request #71 from samtools/feature/output-option

Add -o FILE / --output FILE option to redirect output
Updated man page
stats: Use lowercase in "number of samples" output
stats: Fix in R^2 calculation

By mistake, RA counts were swapped with AA counts. The calculation
now made more clear to read.
call: Support for symbolic <X> allele
call: Symbolic <X> allele to work with -V
norm: Do not try normalize symbolic alt alleles
stats: Be explicit, do not assume specific values of GT_ macros
Merge branch 'develop' into RoH