Git now ignores `/vendor` and the `rouge-memory.tmp` file.

When `Lexer.debug_enabled?` returns `true`, `bool_option(:debug)` was 
causing a fresh instance of the 'debug' string to be allocated. This    
commit avoids this by passing a `String` literal instead of a `Symbol`.

Version 3.4.0 of Rouge moved from passing `opts[:continue]` to the class
method `.lex` to calling the new class method `.continue_lex`. As
implemented, the change was breaking and could cause Rouge to crash. 
This was not intentional and this commit reverts the change.
Internally, Rouge still uses `.continue_lex` but `.lex` will function as it
did in version 3.3.0. A warning is displayed for uses with `$VERBOSE` set
to true.