Refactor exec package

[fho]

• do not add any characters (newlines) to the command output that is returned, output should not be modified
• allow to specify io.Writers for stdout and stderr output of the command, print the output of the command to those instead of to the debug function
• only record the last N lines for output that is returned as part of the error when something fails, this prevents that we have to buffer everything a commands print, which can require a lot of memory
• when "baur run -o" is executed, show the stdout and stderr output of tasks in different colors