add `label_separator` option

Added `label_separator` option for purpose to change a separator of label. As a default, `:` is used for a label separator.

## example configuration

```
configure 'sample' do
  output STDOUT
  rate 500
  delimiter "\t"
  labeled true
  label_separator "-"
  field :id, type: :integer, countup: true, format: "%04d"
  field :time, type: :datetime, format: "[%Y-%m-%d %H:%M:%S]", random: false
  field :level, type: :string, any: %w[DEBUG INFO WARN ERROR]
  field :method, type: :string, any: %w[GET POST PUT]
  field :uri, type: :string, any: %w[/api/v1/people /api/v1/textdata /api/v1/messages]
  field :reqtime, type: :float, range: 0.1..5.0
  field :foobar, type: :string, length: 8
end
```

## output

`$ ./bin/dummer -c example/sample-labeled.conf`

```
id-0234 time-[2014-04-11 16:17:59]      level-ERROR     method-PUT      uri-/api/v1/people      reqtime-2.6887441423503167      foobar-qFQVvH3k
id-0235 time-[2014-04-11 16:17:59]      level-DEBUG     method-PUT      uri-/api/v1/messages    reqtime-3.411200515930845       foobar-NHS0S2E6
id-0236 time-[2014-04-11 16:17:59]      level-ERROR     method-PUT      uri-/api/v1/people      reqtime-4.3141302024983 foobar-geTu6Em7
id-0237 time-[2014-04-11 16:17:59]      level-INFO      method-POST     uri-/api/v1/messages    reqtime-2.5077623430663483      foobar-4cAUznB5
id-0238 time-[2014-04-11 16:17:59]      level-INFO      method-POST     uri-/api/v1/textdata    reqtime-0.26060192871914767     foobar-f5Xzm0gV
```