Fix/numeric subject case

[byCedric]

Description

This fixes #438

Motivation and Context

I've added edge case-ignores to both @commitlint/rules's subject-case rule as well as @commitlint/ensure's case. The reason for putting ignores in 2 places are these.

1. You said that we should ignore all characters not matching /^[a-z]/i from the case rules in general.
   This was my initial thought of putting an ignore/exception into the /ensure package. Unfortunately, this conflicted with the test of LOWERCASE (with backtick), so I had to change it to ignore /^[0-9]/.

2. In the subject case you can negate the rule
   Due to the negation happening in the /rules package, and not /ensure, I had to also include the ignore here. I've tested it with both the !a-z as well as 0-9 ignore pattern, and here we actually could use the !a-z exclusion rule.

• subjectCase('1.0.0', 'never', 'lowercase') === true
• subjectCase('1.0.0', 'always', 'lowercase') === true

Usage examples

$ printf "chore(release): 1.0.1 [skip ci]\n\n## [1.0.1](https://github.com/xmlking/ngx-starter-kit/compare/v1.0.0...v1.0.1) (2018-09-09)\n\n\n### Bug Fixes\n\n* **build:** now updating version ([2d913ea](https://github.com/xmlking/ngx-s
tarter-kit/commit/2d913ea))\n\n\n\n" | npx commitlint -x '@commitlint/config-conventional'

How Has This Been Tested?

See motivation & context

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[marionebl]

Thanks! <3