feat: add support for rendering .Body after .Subject as part of list (#121)

When attempting to render a commit body below the summary line of the
commit there are two problems:

1) The text needs to be indented two spaces to appear as part of the
list.

2) Notes (e.g. BREAKING CHANGE) are included in the body and end up
being repeating in a Notes section (if this is part of your template).

To address #1 add an `indent` func to the template parsing.
To address #2 add a `TrimmedBody` to the `Commit` fields.

The `TrimmedBody` will include everything in `Body` but not any
`Ref`s, `Note`s, `Mention`s, `CoAuthors`, or `Signers`.

Both the CoAuthors and Signers are now first class in the Commit
struct.

With both of these a template block like:

```
{{ if .TrimmedBody -}}
{{ indent .TrimmedBody 2 }}
{{ end -}}
```

Will render the trimmed down body section as intended.

See TestGeneratorWithTimmedBody for example of desired output.

chglog_test.go

@@ -463,3 +463,82 @@ func TestGeneratorWithTagFiler(t *testing.T) {
 [Unreleased]: https://github.com/git-chglog/git-chglog/compare/v1.0.0...HEAD`, strings.TrimSpace(buf.String()))
 
 }
+
+func TestGeneratorWithTimmedBody(t *testing.T) {
+	assert := assert.New(t)
+	testName := "trimmed_body"
+
+	setup(testName, func(commit commitFunc, tag tagFunc, _ gitcmd.Client) {
+		commit("2018-01-01 00:00:00", "feat: single line commit", "")
+		commit("2018-01-01 00:01:00", "feat: multi-line commit", `
+More details about the change and why it went in.
+
+BREAKING CHANGE:
+
+When using .TrimmedBody Notes are not included and can only appear in the Notes section.
+
+Signed-off-by: First Last <first.last@mail.com>
+
+Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>`)
+
+		commit("2018-01-01 00:00:00", "feat: another single line commit", "")
+		tag("1.0.0")
+	})
+
+	gen := NewGenerator(NewLogger(os.Stdout, os.Stderr, false, true),
+		&Config{
+			Bin:        "git",
+			WorkingDir: filepath.Join(testRepoRoot, testName),
+			Template:   filepath.Join(cwd, "testdata", testName+".md"),
+			Info: &Info{
+				Title:         "CHANGELOG Example",
+				RepositoryURL: "https://github.com/git-chglog/git-chglog",
+			},
+			Options: &Options{
+				CommitFilters: map[string][]string{
+					"Type": {
+						"feat",
+					},
+				},
+				CommitSortBy:      "Scope",
+				CommitGroupBy:     "Type",
+				CommitGroupSortBy: "Title",
+				CommitGroupTitleMaps: map[string]string{
+					"feat": "Features",
+				},
+				HeaderPattern: "^(\\w*)(?:\\(([\\w\\$\\.\\-\\*\\s]*)\\))?\\:\\s(.*)$",
+				HeaderPatternMaps: []string{
+					"Type",
+					"Scope",
+					"Subject",
+				},
+				NoteKeywords: []string{
+					"BREAKING CHANGE",
+				},
+			},
+		})
+
+	buf := &bytes.Buffer{}
+	err := gen.Generate(buf, "")
+	output := strings.ReplaceAll(strings.TrimSpace(buf.String()), "\r\n", "\n")
+
+	assert.Nil(err)
+	assert.Equal(`<a name="unreleased"></a>
+## [Unreleased]
+
+
+<a name="1.0.0"></a>
+## 1.0.0 - 2018-01-01
+### Features
+- another single line commit
+- multi-line commit
+  More details about the change and why it went in.
+- single line commit
+
+### BREAKING CHANGE
+
+When using .TrimmedBody Notes are not included and can only appear in the Notes section.
+
+
+[Unreleased]: https://github.com/git-chglog/git-chglog/compare/1.0.0...HEAD`, output)
+}