Add support for build command arguments to the build html action

[briantist]

The build action takes an input for the path to the build command, and defers actual building to that command.

Since paths can contain spaces, the path is quoted when invoked.

But that means you can't give arguments to the build command, because they'll end up inside the quotes.

A new input for arguments would allow for more flexibility.

For example, I wanted to use this in a different project that has a Makefile for building, and I wanted to give arguments to the make command, but couldn't do so. Instead I committed a build.sh to call make with the arguments.