Refactor generation code and allow custom sections

There's a lot in this PR.
- Added a Section class to more easily make the other changes and
  hopefully add flexibility for the future
- Added an option called `configure_sections` that allows you create
  your own custom sections. It blows away all other sections and uses
only the ones you give it.
- Added an option called `add_sections` that allows you to add_sections
  to the default section set
- Added an option called `include_merged` that can be used when
  configure_sections is defined. Configure sections blows away any and
all default sections so to get this one back, you have to set this
option.
- Added tests for this stuff

@HAIL9000 was a co-author. Because of a little git snafu, I accidentally
squashed all of our work into one so it looks like it was just me.

---

Refactor details:

Before this change, the code in generator.rb and generator_generation.rb was conflated and
method call flow went back and forth between the two files seemingly
randomly. They also both defined the exact same class, which is
un-ruby-ish. I tried to separate methods used for the whole changelog
generation from methods used for specific parts of the changelog and
move them into specific classes.

I reasoned that a changelog is a series of "entries" of all tagged
releases plus an extra entry for the unreleased entry. Each entry is
comprised of a header and a series of "sections" for that entry. Each
section is comprized of a list of issues and/or pull requests for that
entry. So the log contains entries, entries contain sections, and
sections contain issues & prs. I have structured the classes around this idea.

- lib/github_changelog_generator/generator/generator.rb is for code
related to generating the entire changelog.
- lib/github_changelog_generator/generator/entry.rb is for code related
to generating entries.
- lib/github_changelog_generator/generator/section.rb is for code
relating to geneating entry sections.

Issues and PRs are already special objects, so it doesn't make sense to
break those out into their own class.

man/git-generate-changelog.1.html

@@ -261,13 +261,21 @@
 
 <p>   Run verbosely. Default is true</p>
 
-<p>  -v, --version</p>
+<p>   --configure-sections [HASH, STRING]</p>
 
-<p>  Print version number</p>
+<p>   Define your own set of sections which overrides all default sections") do |v|</p>
 
-<p>  -h, --help</p>
+<p>   --add-sections [HASH, STRING]</p>
 
-<p>  Displays Help</p>
+<p>   Add new sections but keep the default sections"</p>
+
+<p>   -v, --version</p>
+
+<p>   Print version number</p>
+
+<p>   -h, --help</p>
+
+<p>   Displays Help</p>
 
 <h2 id="EXAMPLES">EXAMPLES</h2>
 
@@ -286,7 +294,7 @@
 
   <ol class='man-decor man-foot man foot'>
     <li class='tl'></li>
-    <li class='tc'>October 2017</li>
+    <li class='tc'>December 2017</li>
     <li class='tr'>git-generate-changelog(1)</li>
   </ol>