[Bug] Seems to break with certain emoji strings

Hey @daltoniam !

I just tried running an emoji-laden string past MDEmphasisPattern, and it seems to crash with an Fatal error: cannot increment beyond endIndex in

bumblebee/Sources/bumblebee.swift

Line 90 in 842fb80

    
           let end = mutText.index(mutText.startIndex, offsetBy: range.location + range.length)

I have a fork with a test case added, but I've messed with a lot of build settings(from xcodegen: https://github.com/yonaskolb/XcodeGen) and don't think you'd want to merge. Check this out if you're interested though: codeOfRobin@62d6c4d

I'll try fixing it in my spare time, but I'd love to hear your thoughts on it!

Here's the code in case you wanna check it out yourself:

let breakingString = " *hello* .  *hello*  cncnc cncnc  *hello*  hmm\n  *hello*  🦌🐎🦌🐎\n  *hello*  Screen Shot 2017-10-12 at 11.04.58 AM.png  *hello*  vVe2keakU8.gif  *hello*   *hello* "


let parser = Parser()

let parseExpectation = expectation(description: "parsing expectation")

parser.add(pattern: MDEmphasisPattern()) { (str, attributes) in
	return MatchedResponse(string: str, attributes: [:])
}

parser.process(text: breakingString) { (attrString) in
	guard let _ = attrString else {
		return
	}
	parseExpectation.fulfill()
}

wait(for: [parseExpectation], timeout: 10.0)

Is fixed in #29 if you'd like to take a look 😄

Hey @codeOfRobin I still had the issue with your PR.
I submitted a new one #31

Interesting @CWalig 🤔. Could you point me to a failing test case so I can check it out?

What about?

func testSingleEmoji() {
		let breakingString = "🐎🐎 *hello* "
   
		let parseExpectation = expectation(description: "parsing expectation")

		parser.add(pattern: MDEmphasisPattern()) { (str, attributes) in
			return MatchedResponse(string: str, attributes: [NSAttributedStringKey.font: UIFont.boldSystemFont(ofSize: 10.0)])
		}

		parser.process(text: breakingString) { (attrString) in
			guard let attrString = attrString else {
				return
			}
			let rangeToCompare = NSRange.init(location: 4, length: 5)
			attrString.enumerateAttributes(in: NSRange(location: 0, length: attrString.string.count), options: [.longestEffectiveRangeNotRequired], using: { (value, range, isStop) in
                if range == rangeToCompare {
					parseExpectation.fulfill()
				}
			})
		}

		wait(for: [parseExpectation], timeout: 10.0)
    }

Let me know if I am missing something 👍

@CWalig I wrote a test case specifically to make sure it worked 🤔 https://github.com/daltoniam/bumblebee/pull/29/files#diff-3c95b3eb97e5e17dbd3db4ad8fa45c2e

I know, this one has a different breakingString and different rangeToCompare.
Have you tried it? 🤔