When working with local files in a plan, it's helpful to be able to verify whether they exist and are readable before trying to use them. For example, setting up a new host might involve copying over whichever of a set of files are present on the local machine. We should have built-in functions for checking the existence and readability of a local file.
Added two new functions bundled with Bolt, {{file::exists}} and {{file::readable}}. These test whether a given file exists and is readable, respectively.