* PHP: Improve operator coloring, support Nowdocs, Heredocs

[earboxer]

I recently started using the font Monoid, which has ligatures for common programming symbols (->, =>, >=, etc). However, my terminal emulator wouldn't connect the ligatures if the syntax highlighting had them in a different color. This inspired me to update the syntax highlighting file.

Features

• boolean and null literals (true, FALSE, null, etc) are now highlighted in their own color (similar to how numbers are)
• Different types of operators have their own colors (bitwise magenta, comparison and logical yellow, assignment brightblue)
• variables are now green (yeah, this is the only subjective change made here that I care much about; since green is associated with dollars)
• braces and arrows are cyan (yes, adding another color to the palette... helps to break up variable access without blending with method calls)

$object->attribute;
$array['key'];
$object->method('attribute');

• Initial support for nowdocs and heredocs (at least those ending in semicolons)
• Also close #286

[scopatz]

Cool, thanks!