Allow disabling of jekyll-feed while in development

[sylhare]

SInce #346 was not getting any traction, I addressed the code review comments and created this PR.

[sylhare]

Oh it seems I need approval to run the workflow, @parkr would you be able to take a look. Thanks!

[sylhare]

Ah it gets:

lib/jekyll-feed/generator.rb:9:5: C: Metrics/AbcSize: Assignment Branch Condition size for generate is too high. [<5, 23, 9> 25.2/21]
    def generate(site) ...
    ^^^^^^^^^^^^^^^^^^
lib/jekyll-feed/generator.rb:9:5: C: Metrics/CyclomaticComplexity: Cyclomatic complexity for generate is too high. [8/7]
    def generate(site) ...
    ^^^^^^^^^^^^^^^^^^

[parkr]

@jekyllbot: merge +minor

[tadamcz]

This change appears in the README on the master branch, but has not been released, which can be confusing. Especially since earlier versions fail silently.

I had to use:

gem 'jekyll-feed', git: "git@github.com:jekyll/jekyll-feed.git"

[parkr]

0.17.0 is out now.