Symbolize option keys

When using a yaml configuration file to initialize Yell loggers, the keys come through as strings instead of symbols (even when using symbolized keys in the yaml). Example configuration file:
```
environment:
  :format: '<%= Yell::ExtendedFormat %>'
  :level: 'gte.debug'
  :adapters:
    - :gelf:
        :host: 'graylog_host.com'
        :port:  12345
        :facility:  'custom_facility'
        :level: 'gte.error'
```

Logging out options in `setup` returns the following:
```
#<Yell::Adapters::Collection:0x00000000000000
  @options={
      "format"=>"%d [%5L] %p %h : %m\n",
      "level"=>"gte.debug",
      "adapters"=>[{
          "gelf"=>{
              "host"=>"graylog_host.com",
              "port"=>12345,
              "facility"=>"custom_facility",
              "level"=>"gte.error"
          }
      }]
  },
  @collection=[#<Yell::Adapters::Gelf level: #<Yell::Level severities: ERROR, FATAL, UNKNOWN>>]
>
```

Compared with creating a logger using Yell.new:
```
#<Yell::Adapters::Collection:0x00000000000000
  @options={
    :host=>"graylog_host.com",
    :port=>12345,
    :facility=>"custom_facility",
    :level=>"gte.error"
  },
  @collection=[#<Yell::Adapters::Gelf level: #<Yell::Level severities: ERROR, FATAL, UNKNOWN>>]>
```

This PR symbolizes the keys of the options hash, allowing the gelf adapter to properly initialize using values in the config file rather than just falling back to the defaults. It doesn't symbolize the gelf hash itself, but from my testing it works fine as long the base keys are symbolized.