Fix broken repl output

[ghalestrilo]

Right now the :repl messages are stored inside the state as an array of strings. the problem is: usually the process output doesn't come buffered onto lines, so when rendered, the text comes off broken. Also, sliding is very rudimentary - a better buffer leads to better sliding.

Options

1. Investigate node's child_process lib, maybe there is a setting for buffering newlines
2. buffer them manually

• Store repl output as a string
• append to this string every evaluation / output (instead of pushing a new string to the array)
• split on \n
• limit to n lines
• join them back together with \n