Make checks DRY (as discussed in #12)

```
Installing dependencies
?   	github.com/rafalchmiel/flint	[no test files]
=== RUN TestCheckReadme
--- PASS: TestCheckReadme (0.00 seconds)
=== RUN TestCheckContributing
--- PASS: TestCheckContributing (0.00 seconds)
=== RUN TestCheckLicense
--- PASS: TestCheckLicense (0.00 seconds)
=== RUN TestCheckBootstrapScript
--- PASS: TestCheckBootstrapScript (0.00 seconds)
=== RUN TestCheckTestScript
--- PASS: TestCheckTestScript (0.00 seconds)
=== RUN TestSeverity
--- PASS: TestSeverity (0.00 seconds)
PASS
ok  	github.com/rafalchmiel/flint/flint	0.049s
```