Firefox SVG support.

Firefox appears to have issues with the standard `svg+xml`, `non-encoded data` combination. I've found the following to work across Chrome and Firefox (IE works, but has other issues when attempting to render any SVG data to a canvas it seems.)

Based on the discussion here:
https://css-tricks.com/probably-dont-base64-svg/#comment-1586152

I've had better luck dropping the encoding altogether:

```
var svgHead = function(i,type) { return i == 0 ? "module.exports = 'data:image/svg+xml," : '' }
```

And leveraging `encodeURIComponent` instead of `esc`.

```
function svgStream(file, type) {
var i = -1
return through(
function (buf, enc, next) { i++; this.push(svgHead(i,type)); this.push(encodeURIComponent(buf.toString('utf-8'))); next() },
function (end) { this.push(svgTail()); end() }
)
}
```

Start by reading the svgHead and svgStream entry points shown in the issue, including the current use of esc and the SVG data-URI construction. Verify the behavior in Firefox and Chrome, then update the encoding approach so SVG data renders consistently and confirm the generated module output works.

Written by the indexing model from the issue text.