Update agnoster to use new powerline codepoints

This merges #1556 into the latest master and adds a configuration variable. By default, agnoster will use the old version of powerline. To use the new version, add `USE_NEW_POWERLINE="true"` to your `.zshrc`.

I also converted tabs to spaces in the `prompt_hg` function definition and updated it to use the configured `BRANCH_SYMBOL`.