renderdag: box: option to pad between nodes from separate branches

[thomasa88]

Makes it easier to visually scan the branches and find the heads quickly.

Before:

│ │ │ o  F some text message
│ ├───╯  that continues on the next line
│ │ │ o  E some text message
│ ├───╯  that continues on the next line
│ │ o  D some text message
│ │ │  that continues on the next line
│ │ o  C some text message
│ │ │  that continues on the next line
│ o │  B some text message
│ ├─╯  that continues on the next line
│ o  A some text message
├─╯  that continues on the next line

After:

│ │ │ o  F some text message
│ ├───╯  that continues on the next line
│ │ │
│ │ │ o  E some text message
│ ├───╯  that continues on the next line
│ │ │
│ │ o  D some text message
│ │ │  that continues on the next line
│ │ o  C some text message
│ │ │  that continues on the next line
│ │ │
│ o │  B some text message
│ ├─╯  that continues on the next line
│ o  A some text message
├─╯  that continues on the next line

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