Add support for multiple config files

[yusufhm]

First pass at the merge; currently it can merge the following scenario.

Given the following files:

# initial-config.yml
checks:
  file:
    - name: '[FILE] Illegal files'
      path: web/sites/default/files
      disallowed-pattern: '^(adminer|phpmyadmin|bigdump)?\.php$'

# config-override.yml
checks:
  file:
    - name: '[FILE] Illegal files'
      path: ./

Running go run . -f initial-config.yml -f config-override.yml --dump-config will provide the following config:

project-dir: /Users/yusuf/projects/shipshape
fail-severity: high
checks:
  file:
    - name: '[FILE] Illegal files'
      severity: normal
      path: ./
      disallowed-pattern: ^(adminer|phpmyadmin|bigdump)?\.php$

Note: I did some major refactoring:

• Broke out the code for each struct in its own file
• Added a check suffix to the check files - a future refactoring would be to move the checks out into a separate directory altogether
• In the tests, I'm moving away from using Ensure* and the like, to instead use assert.*

Still a work in progress, and the next step is to allow the following config override to work:

checks:
  file:
    - path: ./

The intention would be to replace the path for all file checks in the config.

[yusufhm]

The merge is now working as required; given the following files:

# initial-config.yml
checks:
  file:
    - name: '[FILE] Illegal files'
      path: web/sites/default/files
      disallowed-pattern: '^(adminer|phpmyadmin|bigdump)?\.php$'

# config-override.yml
checks:
  file:
    - path: ./

Running go run . -f initial-config.yml -f config-override.yml --dump-config will produce the following config:

project-dir: /Users/yusuf/projects/shipshape
fail-severity: high
checks:
  file:
    - name: '[FILE] Illegal files'
      severity: normal
      path: ./
      disallowed-pattern: ^(adminer|phpmyadmin|bigdump)?\.php$