Allow CompletionItem to control how text is being inserted

[jrieken]

Today, when inserting completion items (and snippets) that span multiple lines we adjust whitespace. That's not always wanted (see microsoft/language-server-protocol#83) and we should expose a flag that controls how the CompletionItem#insertText is being inserted.

In Roslyn, CompletionItemRules exist and we should either have something comparable or inline a property ala "insertAsIs"

[jrieken]

This will be finalised in September.