adds default config to enable all rules work out of the box

[chavacava]

Closes #749
By adding (very opinionated) defaults to the following rules:

1. argument-limit (8)
2. banned-characters (none)
3. cognitive-complexity (7)
4. cyclomatic (10)
5. file-header (none)
6. function-length (50 statements, 75 lines)
7. function-result-limit (3)
8. line-length-limit (80)
9. max-public-structs (5)

These defaults will allow to run revive with a minimal configuration

ignoreGeneratedHeader = false
severity = "warning"
confidence = 0.8
errorCode = 0
warningCode = 0
enableAllRules = true

[mgechev]

I'm thinking that we should probably document the default values in the README. Otherwise LGTM!