Fix for render() being called with node.js buffer instead of string

hi,

attempts to fix render() being called with a node.js buffer object instead of a string, which results in the following error:

```
$ aglio -i README.md -o output.html

/usr/local/lib/node_modules/aglio/lib/main.js:79
    filteredInput = !options.filterInput ? input : input.replace(/\r\n?/g, '\n
                                                         ^
TypeError: Object FORMAT: 1A

# SalesForce Connector API

The Salesforce FeedHenry Connector API.

# SalesForce Connector API Root [/]

SalesForce API entry point.

This resource does not have any attributes. Instead it offers the initial API affordances in the form of the HTTP Link header and HAL links.

## Retrieve Entry Point [GET]

+ Response 200 (application/hal+json)
    + Headers

            Link: <http:/api.gistfox.com/>;rel="self",<http:/api.gistfox.com/gists>;rel="gists"

    + Body

            {
                "_links": {
                    "self": { "href": "/" },
                    "gists": { "href": "/gists?{since}", "templated": true }
                }
            } has no method 'replace'
    at Object.exports.render (/usr/local/lib/node_modules/aglio/lib/main.js:79:58)
    at exports.renderFile (/usr/local/lib/node_modules/aglio/lib/main.js:121:22)
    at fs.readFile (fs.js:176:14)
    at Object.oncomplete (fs.js:297:15)
```