feat: base path for relative file uris

fixes #13
I addressed clkao's concerns that the original change would be breaking for absolute file uris.

The file base path is good in most cases. However when using a glob pattern to grab files it is helpful for the base option to be relative to the file streams current base path.

For example
When using a glob pattern foo/**/*.html
With a file foo/bar/biz/baz.html

You get a file stream of:
base: foo/
path: foo/bar/biz/baz.html

In this case passing a base option of bar/biz/ will leave you with just baz.html when generating the template cache.