Add support for git-credential-helper
    
    This patch adds an additional field `options` to scm_dict, which can be
    used to provide additional information to the backends.
    
    It implements a single new option for GitWrapper. This option allows
    setting a custom git credentials wrapper. This can be useful if Pungi
    needs to get files from a git repository that requires authentication.
    
    The helper can be as simple as this (assuming the username is already
    provided in the url):
    
        #!/bin/sh
        echo password=i-am-secret
    
    The helper would need to be referenced by an absolute path from the
    pungi configuration, or prefixed with ! to have git interpret it as a
    shell script and look it up in PATH.
    
    See https://git-scm.com/docs/gitcredentials for more details.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>
    JIRA: RHELCMP-11808