Support faster template engine to improve the performance

[suzuki-shunsuke]

Feature Overview

Now aqua uses Go's text/template.
But I guess we can use simpler and faster template engine.

Why is the feature needed?

To improve the performance.

Example Code

Configuration

asset: foo-{{.OS}}-{{.Arch}}.{{.Format}}
asset_template_engine: simple

Note

To reduce the migration cost and keep the compatibility, we consider the subset of Go's text/template, which merely replaces placeholders like {{.OS}} with values.
In most cases, it is sufficient.

This change doesn't break the compatibility.
Old aqua still works fine.