Fix incorrect `messageFormatter` type

[astorije]

Fixes https://github.com/RoccoC/webpack-build-notifier/pull/68

It looks like I messed up a rebase or something. Sorry!

(It would be nice if the types were not defined twice as I've hit this a couple times in my PRs. I can take a stab at consolidating them from the Config object if you're interested)

[coveralls]

Pull Request Test Coverage Report for Build 155

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.636%

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Totals
Change from base Build 151:	0.0%
Covered Lines:	130
Relevant Lines:	134

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💛 - Coveralls

[RoccoC]

Whoops! I missed it, too.

Yeah, would be better if that type wasn't defined twice. I'll merge your PR and will consolidate the types.

[RoccoC]

Done: ba29e0dca2b341cc65fa541a45d435fe4f877776

Although it would probably have been cleaner to just collapse all of the individual class properties into one config object, e.g.:

before:

export default class WebpackBuildNotifierPlugin {
  // config options
  private title: string = 'Webpack Build';
  private logo?: string;
  private sound: string = 'Submarine';
  private successSound: string;
  private warningSound: string;
  ...

after:

export default class WebpackBuildNotifierPlugin {
  private config: Config;
...

Maybe an improvement for another time. :)

[RoccoC]

Thanks for the fix!