[volt get] When clone/fetch/pull is failed, should try to run git command

[tyru]

Problem

Cannot install plugin using relative submodule path like kana/vim-repeat. #187

Solution (Workaround?)

Add config value get.fallback_git_cmd.

• true (default): When "volt get" or "volt get -u" fail and "git" command is
  installed, it tries to execute "git clone" or "git pull" as a fallback
• false: "volt get" or "volt get -u" won't try to execute fallback commands