Add new Git commit message template: bump version

dotfiles/gittum/default/msg_bumpv

+Bump version

dotfiles/gittum/main.gitconfig

         'git commit --edit --message=\"$(~/.gittum/bin/findcat msg_flake8)\"'
   cmt = !sh -c \
         'git commit --edit --message=\"$(~/.gittum/bin/findcat msg_typo)\"'
+  cmv = !sh -c \
+        'git commit --edit --message=\"$(~/.gittum/bin/findcat msg_bumpv)\"'
   cmw = !sh -c \
         'git commit --edit --message=\"$(~/.gittum/bin/findcat msg_wip)\"'
   co = checkout