fix(rules): export subject-min-length rule

[whizark]

Hi, subject-min-length rule in [email protected] has been missing since 80cd7cc.

source/rules/index.js should export source/rules/subject-min-length.js 😃

my-sharable-config/index.js:

module.exports = {
    rules: {
        'subject-min-length': [
            1,
            'always',
            0
        ]
    }
};

Stack trace:

/home/user/project/node_modules/conventional-changelog-lint/distribution/cli.js:235
                throw err;
                ^

TypeError: rule is not a function
    at /home/user/project/node_modules/conventional-changelog-lint/distribution/index.js:133:20
    at Array.map (native)
    at _callee$ (/home/user/project/node_modules/conventional-changelog-lint/distribution/index.js:112:10)
    at tryCatch (/home/user/project/node_modules/regenerator-runtime/runtime.js:64:40)
    at Generator.invoke [as _invoke] (/home/user/project/node_modules/regenerator-runtime/runtime.js:299:22)
    at Generator.prototype.(anonymous function) [as next] (/home/user/project/node_modules/regenerator-runtime/runtime.js:116:21)
    at step (/home/user/project/node_modules/babel-runtime/helpers/asyncToGenerator.js:17:30)
    at /home/user/project/node_modules/babel-runtime/helpers/asyncToGenerator.js:35:14
    at F (/home/user/project/node_modules/core-js/library/modules/_export.js:35:28)
    at /home/user/project/node_modules/babel-runtime/helpers/asyncToGenerator.js:14:12

[marionebl]

Thanks for contributing - it's very late where I live and I don't have access to a full working machine right now, so I'll release in approx. 12 hours