Added new CLI and convertFile method to API

[neocotic]

This PR delivers what was requested in #2.

I've added a new CLI which has the following usage:

Usage: convert-svg-to-png [options] [files...]


  Options:

    -V, --version              output the version number
    --no-color                 disables color output
    -b, --base-url <url>       specify base URL to use for all relative URLs in SVG
    -f, --filename <filename>  specify name the for target PNG file when processing STDIN
    --height <value>           specify height for PNG
    --width <value>            specify width for PNG
    -h, --help                 output usage information

I've also added a new convertFile method to the API which is utilized by the CLI but, I believe, will also be useful to consumers.

CLI

The CLI can work in the following ways:

• SVG file paths are specified and are converted to a new PNG file placed in the same directory, where the file name is the same as the original except that the file extension, whatever it is (we don't care if it's .svg/.pony), is replaced with .png
  • Glob patterns can be specified in quotes for cross-platform globbing
  • URLs created from each file path is used as the default base URLs
• SVG is piped to STDIN:
  • If -f, --filename option is specified, the resulting PNG buffer will be written to that file
  • Otherwise, the PNG buffer will be piped to STDOUT
  • process.cwd() is used as default base URL

The -b, --base-url option will always take precedence over the defaults mentioned above.

convertFile(sourceFilePath[, options])

The new convertFile API method is a simple wrapper around the existing convert method that reads the SVG buffer from a specified file path and writes the converted PNG buffer to another file path. It takes the same options with the additional of a targetFilePath option, which can be specified to control where the PNG buffer is written. If omitted, the target file will be derived from sourceFilePath in the same way as described in the first point of the CLI section above.

It's also worth noting that, if the baseFile and baseUrl options are both omitted, a file URL created from sourceFilePath will be used as the base URL instead of defaulting to process.cwd() as is done for the standard convert method.