Add in support for UUID substitution

[gtrevg]

Updates to cover #6

[sonots]

Thanks, but could you fix followings?

1. Travis is failing. It seems you forgot to add uuidtools gem on gemspec file. See https://github.com/tagomoris/fluent-mixin-config-placeholders/blob/master/fluent-mixin-config-placeholders.gemspec#L18
2. Need tests for uuid placeholders
3. Need to only generate the needed UUID if that variable were passed in, otherwise, this codes make speed slower, and affect people who do not need to use UUID.

[gtrevg]

To run / update unit tests, would that be modifying the spec/out_record_reformer_spec.rb file?

Thanks

[sonots]

@gtrevg Yes

[gtrevg]

Will do.

[sonots]

Let me close this pull request once because it looks there is no progress. The issue is kept open.

[gtrevg]

No problem. I think you've also made some other changes, so I'll just fork a fresh copy and will do a pull request of a more concise update.

Thanks,