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.

fields.go

@@ -8,6 +8,12 @@ type Hash struct {
 	Short string
 }
 
+// Contact of co-authors and signers
+type Contact struct {
+	Name  string
+	Email string
+}
+
 // Author of commit
 type Author struct {
 	Name  string
@@ -70,6 +76,8 @@ type Commit struct {
 	Refs        []*Ref
 	Notes       []*Note
 	Mentions    []string   // Name of the user included in the commit header or body
+	CoAuthors   []Contact  // (e.g. `Co-authored-by: user <user@email>`)
+	Signers     []Contact  // (e.g. `Signed-off-by: user <user@email>`)
 	JiraIssue   *JiraIssue // If no issue id found in header, `nil` is assigned
 	Header      string     // (e.g. `feat(core)[RNWY-310]: Add new feature`)
 	Type        string     // (e.g. `feat`)
@@ -77,6 +85,7 @@ type Commit struct {
 	Subject     string     // (e.g. `Add new feature`)
 	JiraIssueID string     // (e.g. `RNWY-310`)
 	Body        string
+	TrimmedBody string // Body without any Notes/Refs/Mentions/CoAuthors/Signers
 }
 
 // CommitGroup is a collection of commits grouped according to the `CommitGroupBy` option