Expose validation MAX_ERRORS as a configuration parameter

[`MAX_ERRORS`](https://github.com/josdejong/jsoneditor/blob/master/src/js/textmode.js#L17) controls how many validation errors are shown in text view, and is currently hardcoded to 3.

Suggestion is to make this a configurable parameter, either a fixed number in constructor, or even changeable on an active view.

Reason: Especially in cases of `anyOf`, `oneOf` the first 3 errors do not necessarily pinpoint to the actual validation problem. In such cases, it would be nice to have an option to either default to a bigger number of errors shown, or provide a way to show more errors on demand.