add support for package-specific make options through pkg_makeopts

Add an optional pkg_makeopts parameter to `poudriere::env` that can contain a hash of `origin/pkgname` to an array of make options.

This makes the template more complex, so the `inline_template` is converted to a template file.

Update README to include a common example of make options, i.e. building apache24 with a mod_php5 module.