Add the ability to set HTTP headers

[csavelief]

Add the ability to set HTTP headers in order to be able to load PDF files from protected APIs.

Example :

<vue-pdf-viewer
    :url="url"
    :httpHeaders="{'X-CSRF-TOKEN': '<your-csrf-token>', 'X-XSRF-TOKEN': '<your-xsrf-token>'}"/>

[csavelief]

This commit does two things :

1. Add a text layer on top of the canvas ;
2. Give the user the ability to find and highlight text fragments. The caller has the ability to target a specific page and set the color of the highlighted text.

Example :

<vue-pdf-viewer
    :selections="[{text: 'dommages aux biens', color: '#ff9704', page: 3}, {text: 'Moselle', color: 'red', page: 3}]"
    :url="url"/>

Output :

Closes issues #2 and #17.