Disabled force_unwrapping, trailing comma, line_length rules

Summary:
References #894

Disabled force unwrapping, but force try and force cast rules are still in effect:
```swift
let k = object.things! // no longer causes a warning
let r = object.fidget as! Spinnable // causes warning
let s = try! object.doDangerousTask() // causes warning
```
Closes https://github.com/Instagram/IGListKit/pull/897

Differential Revision: D5651127

Pulled By: rnystrom

fbshipit-source-id: 7174cc52d24de3c2cdc15a29c7b053b12d68b1e3

Examples/.swiftlint.yml

@@ -1,5 +1,9 @@
 opt_in_rules: # some rules are only opt-in
   - empty_count
+disabled_rules:
+  - force_unwrapping
+  - comma
+  - line_length
 included:
   - IGListKitExamples
   - IGListKitMessageExample
@@ -9,7 +13,6 @@ excluded:
 force_cast: warning
 force_try: warning
 weak_delegate: error
-line_length: 140
 type_body_length:
   warning: 300
   error: 400