wasm: close base64 encoder (#22)

Close the base64 encoder before returning the buffer. This ensures that
all the bytes are written to the buffer. Without this, the last few
bytes may be missing, resulting in a corrupted image. Usually, this
would only corrupt the checksum, and the image would still be displayed
fine. However, some applications do not like this (e.g., slack).

release: add goreleaser configuration (#17)