Add an option to override version prefix (~ubuntu)

Hi,

sometimes it is important to have a larger version number than in the target release, so an option to override (~ubuntu or ~debian) to something larger might be needed.

Here's the patch that adds an -p,--prefix option to backportpackage:
```
--- /usr/bin/backportpackage 2017-06-13 11:14:14.000000000 +0200
+++ /home/ondrej/bin/backportpackage 2017-07-06 14:13:37.022244568 +0200
@@ -83,6 +83,10 @@ def parse(args):
                       metavar='SUFFIX',
                       help='Suffix to append to version number '
                            '(default: ~ppa1 when uploading to a PPA)')
+ parser.add_option('-p', '--prefix',
+ metavar='PREFIX',
+ help='Prefix to prepend to version number '
+ '(default: ~<distribution> (debian or ubuntu)')
     parser.add_option('-b', '--build',
                       default=False,
                       action='store_true',
@@ -221,14 +225,16 @@ def find_package(mirror, workdir, packag
     return srcpkg

-def get_backport_version(version, suffix, upload, release):
+def get_backport_version(version, suffix, upload, release, prefix):
     distribution = codename_to_distribution(release)
     if not distribution:
         error('Unknown release codename %s' % release)
     series = Distribution(distribution.lower()).\
         getSeries(name_or_version=release)

- backport_version = version + ('~%s%s.1' % (distribution.lower(), series.version))
+ if prefix is None:
+ prefix = '~%s' % (distribution.lower())
+ backport_version = version + ('%s%s.1' % (prefix, series.version))
     if suffix is not None:
         backport_version += suffix
     elif upload and upload.startswith('ppa:'):
@@ -308,7 +314,7 @@ def orig_needed(upload, workdir, pkg):

 def do_backport(workdir, pkg, suffix, close, release, release_pocket, build,
- builder, update, upload, keyid, prompt):
+ builder, update, upload, keyid, prompt, prefix):
     dirname = '%s-%s' % (pkg.source, release)
     srcdir = os.path.join(workdir, dirname)

@@ -321,7 +327,7 @@ def do_backport(workdir, pkg, suffix, cl
     pkg.unpack(dirname)

     bp_version = get_backport_version(pkg.version.full_version, suffix,
- upload, release)
+ upload, release, prefix)
     old_version = get_old_version(pkg.source, release)
     bp_dist = get_backport_dist(release, release_pocket)

@@ -413,7 +419,8 @@ def main(args):
                         opts.update,
                         opts.upload,
                         opts.keyid,
- opts.prompt)
+ opts.prompt,
+ opts.prefix)
     except DownloadError as e:
         error(str(e))
     finally:
```