Add a custom log formatter class

[Pennycook]

Makes the following two changes to the log format:

• Use lower-case prefixes to mimic compiler behavior.
• Use ASCII color codes when available.

Related issues

Closes #25.

Proposed changes

• Add a new Formatter class that applies the desired formatting to our logs.
• Disable colors when writing to file, because ASCII codes don't work there.
• Disable colors when stdout is redirected to a file.

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We might want to revisit this much later on if we want to be really fancy (i.e., if we want to do things like allow warning messages to highlight different parts of their own messages, etc) but the difference from just providing a little bit of color is pretty striking:

[laserkelvin]

Potentially out of scope for this PR since messages aren't defined here, but reading the example I think it would be good to either bold or allocate a new color to differentiate between user/system libraries, i.e. make "user" and "system" stick out more.

[Pennycook]

Potentially out of scope for this PR since messages aren't defined here, but reading the example I think it would be good to either bold or allocate a new color to differentiate between user/system libraries, i.e. make "user" and "system" stick out more.

Yeah, I agree it would be nice to do this.

It's going to require more work, though, as we'll need to come up with a way to filter out any ASCII codes that might appear as part of a message. We'd probably want to come up with (or adopt) some standardized representation of colors and things for the messages. Right now we don't have to worry about that, because the only place these special codes can appear is inside of the logger implementation.

We should open an issue to track it.