What does this do / why do we need it?

this commit addresses the first part of #261 by allowing the passing of arbitrary key=value arguments into the command line and exposing them as part of RenderData for use in the templates

How this PR fixes the problem?

A new --arg CLI parameter has been added. This supports multiple key=value args like so

git-chglog --arg key1=val1 --arg key2=val2 1.0.0

Those --arg values are collected into a slice, then each item split by = into a key and a value, then added into a map and exposed on the RenderData struct so that they can be referenced in templates like this:

{{ index .Args "key1" }} - {{ index .Args "key2" }}

which renders

val1 - val2

Check lists

• Test passed
• Coding style (indentation, etc)

Additional Comments (if any)

None.

Which issue(s) does this PR fix?

fixes #261