feat: reading multiple pdf files with a single PDFParser object

[nicolabaesso]

Elements changed:

1. Added new test case in a separate file
2. Added the two example PDFs
3. Add the reset of the pages array when the data variable is null

I've added this elements because in my corporate job we are using this library, and recreating everytime the PDFParser object is not something I'm a fan of.
Other test cases are not failing, so no regressions.

[modesty]

thanks for adding more tests. A few thoughts on making the instance of PDFJSClass reusable:

1. pdfparser instance (or the client that instantiates PDFJSClass) needs to be reset/reusable whenever PDFParser is created. (line 107 of pdfparser.js)
2. lib/pdf.js: setting this.pages=[] is not sufficient to dispose the object, pdfDocument and rawTextContents needs reset too. Recomment to call existing destroy method.

[nicolabaesso]

Hi @modesty,
thank you for your review. As suggested, I removed the this.pages=[] line and instead called the already available destroy() function.
Also I've added a function to reset the PDFJS object, is this what you were mentioning? Otherwise let me know.
I've also removed the if in line 120 of pdfparser.js, it was a leftover of one test I was doing to understand the code.

[nicolabaesso]

Hi @modesty,
sorry for the pressure, can you give me a feedback on the code? The next days I could make some changes if something is still wrong (I feel like the function for resetting the PDFJS object could use some more work, but I would like to have your opinion).

Thank you!

[modesty]

sorry for the delay. code LGTM. two notes:

1. could you add a command line option to enable it optionally? default false would keep current clients intact.
2. could you merge master into this branch and run all tests when ready?

[nicolabaesso]

sorry for the delay. code LGTM. two notes:

1. could you add a command line option to enable it optionally? default `false` would keep current clients intact.

2. could you merge master into this branch and run all tests when ready?

1. Sorry, I'm not getting it: how we can provide this feature when using the library via command line? Should it be a whole section where, if the flag is provided, you can pass two or more files and the library reads them with a single instance? Just asking

2. Master branch merged, I modified the reset part by calling a similar method to the destroy() one, with the exception of not removing the listeners. Keeping the same method hanged the test forever, with this change the test cases are ok and done in less than 5 seconds.

Let me know what to do next, I'll do it as soon as I can.

Thank you!