add emoji to log level name for iOS

[king6cong]

because we can't tweak Xcode 8 to use color in console
robbiehanson/XcodeColors#88

[king6cong]

[sfackler]

This seems like something an individual logging implementation should do, not the base log crate. This will also break parsing since LOG_LEVEL_NAMES is used for that as well.

[king6cong]

@sfackler Yeah, agree.

Do you think https://github.com/sfackler/log4rs/blob/master/src/encode/pattern/mod.rs#L553
will be a suitable place to handle this?

please also check this estk/log4rs#44 😄

[king6cong]

Updated, only add method to get emoji.
Not sure if this should be in this base log crate,
but I think it may be useful for multiple log implementations
Usage example like this:
estk/log4rs#45

[sfackler]

How standard are these choices of emoji?

[king6cong]

It is compatible with https://github.com/SwiftyBeaver/SwiftyBeaver

[dtolnay]

This doesn't need to be provided in log. I agree with @​sfackler that individual logging implementations can select emojis if they want.

As far as I can tell there isn't a standard but ❌⚠️ℹ️ is probably the most widely recognizable set and works way better in monochrome.

[ThuggishNuggets]

Just a heads up, if you use this in your logging in Xcode you'll get some pretty buggy behaviors trying to use debugger commands as well as some pretty serious performance hits on the logging since Xcode's console text layout engine doesn't seem to like rendering emoji with fixed width fonts.

One bug that's really annoying with this is the autocomplete menus keep flickering and disappearing when using emojis in logging statements.

We ended up removing them from our implementation it was so bad, but YMMV.