Allow user to optionally specify branch when installing from git

[tomkinsc]

Currently, the easy-deploy script has an option, setup-git, to install a copy of viral-ngs from git. It would be helpful for this to accept an argument for a specific branch to check out. Additionally, it could be helpful to add the viral-ngs directory to the path, either directly or by symlinking the viral-ngs *.py files to the bin/ directory of the viral-ngs conda environment.