Enable to use frolint command in no git repository

[yamadayuki]

WHY & WHAT

Under the previous implementation, we cannot use the frolint command without Git.
So we support --no-git option and --files option. And if you install frolint into non git project, frolint uses non git implementation. Result of sample directory is below:

# This project has no .git directory
~/dev/src/github.com/yamadayuki/sample
❯ git status
fatal: not a git repository (or any of the parent directories): .git

# Specify the file to analyze
~/dev/src/github.com/yamadayuki/sample
❯ yarn frolint -f ./index.js
yarn run v1.13.0
$ /Users/yamadayuki/dev/src/github.com/yamadayuki/sample/node_modules/.bin/frolint -f ./index.js
Detected 1 error, 0 warnings
./index.js: 1 error, 0 warnings found.
  ./index.js:1:7 'a' is assigned a value but never used. Allowed unused vars must match /^_/. (no-unused-vars)
✨  Done in 2.27s.

# All js / ts files
~/dev/src/github.com/yamadayuki/sample
❯ yarn frolint
yarn run v1.13.0
$ /Users/yamadayuki/dev/src/github.com/yamadayuki/sample/node_modules/.bin/frolint
Warning: React version not specified in eslint-plugin-react settings. See https://github.com/yannickcr/eslint-plugin-react#configuration .
Detected 2 errors, 0 warnings
./test.js: 1 error, 0 warnings found.
  ./test.js:1:7 'b' is assigned a value but never used. Allowed unused vars must match /^_/. (no-unused-vars)
./index.js: 1 error, 0 warnings found.
  ./index.js:1:7 'a' is assigned a value but never used. Allowed unused vars must match /^_/. (no-unused-vars)
✨  Done in 2.27s.