Add listen new config param "minimum_level" error log.

[Yozhef]

Hello everybody.

Problem:
In the project, I am currently working on it. There is a createRollbarHandler function in the RollbarHandlerFactory class. This LogLevel parameter accepts only Error / Critical values. It would be great if it is possible to put it. Because info info includes Warning / Warning / Error / Critical.

Solution for the problem:
I created you PR on rollbar-php when i add new param in config minimum_level, and this i use him and determine which error level is required.

Example config:

rollbar:
    minimum_level: 'warning'

[ArturMoczulski]

Same here, I recommend using monolog_minimum_level for the config option name as suggested in rollbar/rollbar-php#440 (review)

Also, I think isset($config['minimum_level']) is needed here as I get PHP notices if minimum_level is not set in rollbar.yaml:

$this->minimumLevel = isset($config['minimum_level']) ? $config['minimum_level'] : LogLevel::ERROR;

[Yozhef]

@ArturMoczulski okay, i change it validation, but I do not like the name of this option. I answer rollbar/rollbar-php#440 (review)

[Yozhef]

@ArturMoczulski I added as you requested, but I think verification is not required because we add minimum_level -> default valuer LogLevel::ERROR.

I think if we add to (default, config) value need on code only get.

1. If a person does not use in his config yaml this params -> аutomatically uses value from default.
2. If a person use on his config this param, he override default values.

[ArturMoczulski]

Let's continue this work in #46