Fix type param constraint. Support defaults.

[milesj]

After updating to the latest, I started seeing this error a lot.

TypeError: Cannot read property 'typeAnnotation' of undefined
    at markTypeAnnotationAsUsed (/Users/Miles/Sites/boost/node_modules/eslint-plugin-typescript/lib/rules/no-unused-vars.js:71:37)
    at markTypeAnnotationAsUsed (/Users/Miles/Sites/boost/node_modules/eslint-plugin-typescript/lib/rules/no-unused-vars.js:126:21)
    at Array.forEach (<anonymous>)
    at markClassOptionsAsUsed (/Users/Miles/Sites/boost/node_modules/eslint-plugin-typescript/lib/rules/no-unused-vars.js:260:44)
    at listeners.(anonymous function).forEach.listener (/Users/Miles/Sites/boost/node_modules/eslint/lib/util/safe-emitter.js:45:58)
    at Array.forEach (<anonymous>)
    at Object.emit (/Users/Miles/Sites/boost/node_modules/eslint/lib/util/safe-emitter.js:45:38)
    at NodeEventGenerator.applySelector (/Users/Miles/Sites/boost/node_modules/eslint/lib/util/node-event-generator.js:251:26)
    at NodeEventGenerator.applySelectors (/Users/Miles/Sites/boost/node_modules/eslint/lib/util/node-event-generator.js:280:22)
    at NodeEventGenerator.enterNode (/Users/Miles/Sites/boost/node_modules/eslint/lib/util/node-event-generator.js:294:14)

Seems like some params have an undefined constraint, so add a check for it.

I also added support for defaults, <T = Foo>, since that doesn't seem to be covered.

[milesj]

@nzakas

[bradzacher]

Thanks for your contribution!