API functions for CLI / REPL usage

I spent few hours to learn the internals of StaticLint.jl. In my mind we would need the following functions:
```julia
using StaticLint 

lintpkg("MyPackage") # for linting an entire package
lintfile("my_file.jl") # for linting a file
lintstr("my string") # for linting a string
```
What you think?