Support bitbucket priorities

[sapr9x3]

Hello.
I was thinking about that is not a bad idea to have an ability to migrate isuues priorities from bitbucket to github.
I did it.
I made labels in my github repository: prio:trivial, prio:minor, prio:major, prio:critical, prio:blocker.

Then I just added to config.py

# We map bitbucket's issue "priority" to github "labels".
PRIORITY_MAPPING = {
    "trivial": "prio:trivial",
    "minor": "prio:minor",
    "major": "prio:major",
    "critical": "prio:critical",
    "blocker": "prio:blocker"
}

and to bitbucket_issues_to_github.py

def map_bpriority_to_glabels(bissue, glabels):
    bpriority = bissue['priority']
    if bpriority in config.PRIORITY_MAPPING:
        glabels.add(config.PRIORITY_MAPPING[bpriority])
...
...
def patch_gissue(gissue, bissue, bexport):
    if gissue['title'] != bissue['title']:
        raise ValueError('Inconsistent issues')

    glabels = set()
    map_bkind_to_glabels(bissue=bissue, glabels=glabels)
    map_bstatus_to_glabels(bissue=bissue, glabels=glabels)
    map_bpriority_to_glabels(bissue=bissue, glabels=glabels)  # <-this  

    gissue_patch = {
        "body": construct_gissue_content(bissue=bissue, bexport=bexport),
        "assignees": map_bassignee_to_gassignees(bissue=bissue),
        "labels": list(glabels),
        "state": map_bstatus_to_gstate(bissue=bissue),
    }
...

I can make a PR if this idea is really good.