Improved install phantomjs

[coatless]

This PR implements the PhantomJS installation check as described in #25 by:

1. Adding an is_phantomjs_installed() function that returns TRUE/FALSE if PhantomJS is installed or not.
2. Modifying the parameter of install_phantomjs() to include force, which controls whether PhantomJS will be installed or not if it already is present on the user's computer.

[wch]

It would be better if install_phantomjs checked not just for the presence of phantomjs, but also the version. If phantomjs is not present or if it is an older version, then install it. (Using force=TRUE still always install it.)

[coatless]

@wch thanks. I'll add in a version check.

[coatless]

@wch I've added in the version check. To do so, I've created three internal functions:

1. phantomjs_cmd_result()
   • Passes its arguments to the phantom_run() function.
   • Captures and stores phantom_run()'s STDOUT information instead of only displaying it on console.
2. phantomjs_version()
   • Retrieves the phantomjs version via path/to/bin/phantom --version obtained under phantomjs_cmd_result()
3. is_phantomjs_version_latest()
   • Compares the installed version with requested version and determines if it is out of date.

[wch]

A few minor things, but otherwise it looks good.

[coatless]

@wch ready for round 3.

[wch]

Slight correction: this won't necessarily install the presently-installed version; it will install the version specified by the version parameter. Also, getting here doesn't necessarily mean that the latest version is installed; it just means that the version is >= to version.

[coatless]

I updated the message to reflect this.

[coatless]

@wch let's try round 4?

[wch]

Thanks!