firebase-functions/logger writes structured logs to stdout and stderr by default, and makes these pretty with ANSI color codes.

When functions-framework intercepts these messages to assign execution IDs, it handles them incorrectly in two ways.

First, when the log emitted on stdout or stderr is already a structured log, we override (ignore) the severity set by the logger. This commit avoids changing severity if it's already set.

Second, when parsing the message and attempting to determine if it's already a json object/structured log, it doesn't handle ANSI escape codes (https://en.wikipedia.org/wiki/ANSI_escape_code) used to control color, so parsing these will fail. This means that severity handling falls back to just looking at whether the message came from stdout or stderr, so debug-level and warn-level logs aren't handled correctly. This commit strips all ANSI escape codes that control terminal color. This is a minor bummer because it's a whole lot less pretty, but color-coding and smarter color coding generally seems less important than correct log-level handling.

Fixes #617.