Add functionality for dynamic includes #93

As per the issue raise #93 and with some amendments to corrects the suggest fix. Variables can now be used with includes so that the contents of the variable is evaluated to a string and then the include statement is evaluated as normal with the value of the string as the file name (without extension or path) to search for.
Add functionality for dynamic includes #93

See previous commit for extended explanation.
Merge branch 'master' of https://github.com/jasper-lyons/ejs

Conflicts:
	ejs.js