Applying the snake_case convention throughout the repo (#967)

* Applying the snake_case convention throughout the repo

- Also removing the file extension where not needed
- Adding {{ }} on a few old pages

* Addressing concerns

- Added {{ }} where they were missed out
- Removed spaces inside {{ }}
- Reverting "file" to "filename" to make it clearer

* Fixing the comments on nc page

pages/common/haxelib.md

@@ -16,4 +16,4 @@
 
 - Install the development version of a library from a Git repository:
 
-`haxelib git {{libname}} {{GIT-URL}}`
+`haxelib git {{libname}} {{git_url}}`