Don't clobber a global _ variable if one exists

[pjungwir]

This is a PR to fix Issue #276 . Now pdfmake tells lodash not to set the global _ variable, so you can use a different version of lodash, or underscore, or whatever.

Btw I'm not sure why there are differences here to the files in build. It seems strange to even have those under source control. I ran gulp to build and test my changes, so I guess that's how they got updated. Let me know if you'd like me to do anything else about those.

[nicoabie]

+1
It should be a warning in the docs otherwise. I've come into strange bugs because of the clobbering of my underscore lib.

[bhericher]

+1 I would like to use pdfmake with Pentaho Ctools, but there is conflict with the _

[bhericher]

I confirm that this PR solves my problem.
For those who are interested : https://github.com/bhericher/TableWithButtons

[lobo-tuerto]

+1

[prostoandrei]

When it will be merged? It's very important issue!

[sankaran85]

whether this issue is fixed ?

[csvan]

Was this included in 0.1.20? The problem still persists on that version.

EDIT: 0.1.20 was released before this merge, so no.