feature: add sprig functions to templete.FuncMap

[jacobsin]

fmap @ chglog.go:307 could add sprig functions in to allow more per-repo changelog template customization on the user end without code changes.

E.g. dict, get and set would allow me dedup commit lines sharing the same #issue

Expected Behavior

In CHANGELOG.tpl.md I should be able to create new dict like

{{ $commitsByIssue := dict }}
{{ if not get $commitsByIssues .Refs[0].Ref }}
...
{{ set $commitsByIssues . }}
{{ end }}

Actual Behavior

panic: template: CHANGELOG.tpl.md:17 function "dict" not defined

Steps to Reproduce (including precondition)

1. Add the above gotemplate snippet ln 20-21 and 24-25 to .chglog/CHANGELOG.tpl.md
2. Run git-chglog

Your Environment

• OS: RHEL7
• git-chglog version: 0.9.1

[clok]

Sprig has been included as a part of the v0.14.0 release. See #131 🎉