headTemplate options should be relative to the caller

Previously you had to pass the headTemplate path relative to where
cjs-umd was installed to rather than where you were defining your
options hash. To reference "/templates", you had to pass "../../templates"
and that's assuming it was installed as a primary dependency and not
a dependency of a dependency.
Merge pull request #1 from jmorrell/options-relative-path

headTemplate options should be relative to the caller