By default hidden files, patterns listed in `.export-ignore` as well as
any files ignored by git are excluded from exports. This behavior has
been made configurable on the CLI using the new flags `--hidden`,
`--ignore-file` and `--no-git`.
This refactors the Context to maintain a list of all the files which
have been processed so far in a chain of embeds. This information is
then used to print a more helpful error message to users of the CLI when
RecursionLimitExceeded is returned.