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

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

Author: thomasa88