Allow a custom Gemfile for local development

[aldesantis]

Some extension developers may want to use their own tools such as Pry or Byebug to develop and debug extensions. However, these shouldn't be installed automatically for all users, since everyone has their preference.

We could recommend adding the following to an extension's Gemfile:

load 'Gemfile-custom'

By ignoring Gemfile-custom in Git, users will essentially have a way to use their preferred debugging tools in the extension.

Related issues

• Provide the base Gemfile for extensions #7, as this may be in the base Gemfile we provide.
• The integration with solidus_cmd.

[elia]

Consider using Gemfile.local which seems to be quite common, or to drop the thing entirely and let maintainers just set BUNDLE_GEMFILE=Gemfile.local, see also rubygems/bundler#183.

[aldesantis]

Gemfile.local sounds good, I think we can just add it to .gitignore and add something like the following to the Gemfile we generated with the template:

load 'Gemfile.local' if File.exists?('Gemfile.local')