Use an indicator to show current level in `git gud levels`

[benthayer]

Right now, we explicitly state the current level, but this output isn't very clean and is somewhat verbose. It would be nice to add an indicator for the current level. This would be a part of #185, but it is already so big, it would make sense to make it a separate issue.

[benthayer]

@sahansk2 You could probably take this on

[sahansk2]

I'll tackle this issue.

[sahansk2]

I'll go ahead and implement this:

$ git gud levels

2. Rampup (rampup)
├─◇ 1. Detaching HEAD (detaching)  
├─◈ 2. Relative References I: Using (^) (relrefs1)
├─◇ 3. Relative References II: Using (~) (relrefs2)
└─◇ 4. Reversing Changes in Git (reversing)