[#64] Improved performance by processing parse tree only once.

#### Before
```bash
➜ elvis git:(jfacorro.64.improve.performance) ✗ time elvis rock --config config/elvis.config
# src/elvis.erl [OK]
# src/elvis_code.erl [OK]
# src/elvis_config.erl [OK]
# src/elvis_git.erl [OK]
# src/elvis_github.erl [OK]
# src/elvis_result.erl [OK]
# src/elvis_style.erl [OK]
# src/elvis_utils.erl [OK]
# src/elvis_webhook.erl [OK]
        1.89 real         1.61 user         0.35 sys
```
#### After
```bash
➜ elvis git:(jfacorro.64.improve.performance) ✗ time ./elvis rock --config config/elvis.config
# src/elvis.erl [OK]
# src/elvis_code.erl [OK]
# src/elvis_config.erl [OK]
# src/elvis_git.erl [OK]
# src/elvis_github.erl [OK]
# src/elvis_result.erl [OK]
# src/elvis_style.erl [OK]
# src/elvis_utils.erl [OK]
# src/elvis_webhook.erl [OK]
        0.62 real         0.56 user         0.11 sys
```