Base64 encode source maps

This prevents some edge cases where encodeURIComponent fails to create a valid
source map.

This particular edge case was identified when trying to create source maps for
the lesshat mixin library.

Signed-off-by: Tim Smart <tim.smart@designworks.co.nz>
Merge pull request #2045 from tim-smart/feature/sourcemaps-base64

Base64 encode source maps