Add CI script to blog-data to check for correct post filenames

[Croydon]

All blog posts should have a filename like this: https://github.com/inexorgame/blog-data/blob/75d712241736d68fe2430d5ad680339efe3a8548/post-example.md#filename

If this is not the case our blog may misbehave like listing the post with title but not actually loading the text.

Our Travis CI should check the filenames of posts and error if filenames are incorrect.

[syn228]

Could I try handling this issue if it's not already taken?

[Croydon]

Sure, go ahead! 😄

[Fohlen]

@syn228 thanks for tackling! If you need branch permissions to toy with our CI please let us know on Riot.