Capitalized names makes illegal index name

[LosD]

If capitalized names is used the configuration, the system tries to create an illegal index name (see elastic/elasticsearch#6736 for an attempt to actually specify these limitations in ElasticSearch).

Luckily, GitHub doesn't care about capitalization, so it is rather easy to work around.

Two easy fixes comes to mind: Document that this will happen, or lowercase the generated index name. If the first option is chosen, it might also be a good idea to refuse initializing the river with uppercase characters.

[mihneadb]

Documenting + maybe an assertion sounds good!