Library guidelines

Review guidelines for building library: https://hexdocs.pm/elixir/main/library-guidelines.html

- Check for possible error inside the library:
For example:
https://github.com/dwyl/gogs/blob/2c55a5be2da1dd2db9844fbf6755566988471d42/lib/gogs.ex#L162-L165
Here we call the `File.read` function without checking for errors. We can add a case statement here to make sure we read the file without any errors or report the error from the library itself to make is easier to debug
see: https://hexdocs.pm/elixir/main/library-guidelines.html#avoid-working-with-invalid-data

- Application configuration
At the moment we are using module constant to defined from environment variable. We had a few issues linked to this and it might be better to let the application using the library define these values in the config file.
see https://hexdocs.pm/elixir/main/library-guidelines.html#avoid-application-configuration

- Rename modules
Create the `Gogs` namespace and use it for other modules:
https://github.com/dwyl/gogs/blob/2c55a5be2da1dd2db9844fbf6755566988471d42/lib/helpers.ex#L1
become `Gogs.Helpers`
see: https://hexdocs.pm/elixir/main/library-guidelines.html#avoid-defining-modules-that-are-not-in-your-namespace

These are the main points that we can update for now.