Enable reloading config from environment

[muru]

For #2, using the environment is more convenient than creating a temporary file for updating config during runtime. So I extracted out the code from init() and made it public.

[CLAassistant]

All committers have signed the CLA.

[jbrendel]

Hello! Thank you for your submission. However, I wonder if/how that works. Generally, the environment of a process cannot be updated after the process has started already. Therefore, re-reading environment variables won't yield any changed results, will they? Or do you suggest that the process somehow changes its own environment variables and then calls UpdateEnv? For example with os.Setenv?

[jbrendel]

Ok, I tested it and it works. It still seems kinda odd that we'd have two calls (one to os.System and one to rlog.UpdateEnv) to accomplish one thing, though. I need to think about this for a bit.

[jbrendel]

I have accepted and merged this PR, updated the examples and docs to reflect your changes. Thank you very much again!

[muru]

Ah, yes, the two calls didn't bother me much since I'd been writing a wrapper around rlog anyway (so I'd have a logger.SetLevel(...) which does os.SetEnv() and rlog.UpdateEnv()). I'd played around with a few ideas:

• splitting up the updateConfigFromFile() so that it would call a function capable of using an io.Reader, and then I could use that new function to take Reader objects from strings I made.
• modifying initialize() so that it would optionally skip reading the file, and try to merge the new config object with the old one
• instead of the current UpdateEnv(), a setter which works just like it but taking a key-value pair of arguments

While trying out the last option, I realized that simply re-using the init() code is the simplest change. If I wanted to update multiple settings at once, then calling a setter multiple times (with the corresponding re-reading of config files, etc.) would be very wasteful, butrlog.UpdateEnv can assimilate all changed settings. All other changes would have been more invasive. 😓