Added support for quickmain to use `log` crate. (Address #165)

[dgrnbrg]

• Added crate feature quickmain_log

When quickmain_log is enabled, the quick_main! entry point writes the terminating error chain to log::error! instead of stdout.

Solves #165.

[Yamakaky]

Hum, not sure it's needed... It's a 5 lines function, it's easy to copy if you want more customization.

[dgrnbrg]

I think that this is really valuable for continuing to mature rust. The quick_main! macro is used all over, because it's essentially an adapter that lets you write natively error-chaining libs with the boilerplate generated for you.

For users like me, our systems make extensive use of logging & error-chain. What this really does is makes it easy to write quick CLI tools and take advantage of error-chain & the logging already supported in the application.

The motivation here is that often, one needs to run applications unattended, and this allows boilerplate-free programs to have the reason they terminated in their log file. So, to me, the reason this is valuable is that it handles the 99% case of users who are using the standard log crate, since the termination reason shows up in the log file, plus it keeps the boilerplate down to zero.

[Yamakaky]

Sorry for the delay. Could you update the files?

[rusterize]

@Yamakaky what do you think about adding a set of functions to log erros (using the log crate) that are added to the ResultExt trait? The functions would be:

loge(): prints the error and the cause chain using error! logw(): same with warn! logi(): same with info! logd(): same with debug! ?

It is often that I want to log some errors as they happen without adding a lot of boilerplate if/match and calls to display_error. The functions would be used like chain_err(..) whenever the user wants to log that error at the spot.

something like:

fun_returning_result().chain_err(|| MyCustomErr("some info')).loge()?