Add built-in variables

[hinell]

I request to add support for TextMate-like variables, e.g.:

TM_SELECTED_TEXT    // The currently selected text or the empty string
TM_CURRENT_LINE     // The contents of the current line
BLOCK_COMMENT_START // Example output: in PHP /* or in HTML <!--
BLOCK_COMMENT_END   // Example output: in PHP */ or in HTML -->
LINE_COMMENT        // Example output: in PHP 
...

See full list of variables here.