Redirect process output directly to screen instead of waiting for line endings?

[seaneagan]

Currently runAsync buffers the output until encountering a line ending, and then passes it to log. log currently just forwards to print, but we will likely want to change that in the future. Some potential changes:

• Colorful output #100
• Support parallel task execution #170 (comment)
• Log level options #101 (comment).

However, some apps use ansi escape sequences to do clever UI enhancements, such as a progress animation, or not actually printing out newlines, but rather replacing the line output once a new line occurs, such as test (see #194). But when grinder calls these processes they will likely eventually (https://dartbug.com/21821) see the stdioType as a pipe, and disable these features anyways. Unless dart will provide some way for a child process to inherit the stdio (stdioType especially) of a parent process (grinder in this case)?

So should runAsync (and it's callers) have a mode which enables these interactive features, and doesn't buffer the output for later consumption by log? Should it be the default?

[devoncarew]

dupe of #290