Manuscript Revision Update

Incorporates changes related to peer review:

• Ability to input user-specified burden weights
• A new "nullstats" flag that can be used to run negative-control analyses with permuted burden statistics, to check for stratification
• A new "intercept" flag that can be used to fix the BHR intercept at 1/N

bhr_initial

Initial release of BHR, accompanying medRxiv preprint.