add key_is_before_tag option, add string_keys_utils to be used like a…

[salvprest]

…n singleton to take advantage of the new logic
This commit add a new option key_is_before_tag, if is checked the string before the tag ($$ for example) is used in the csv like a key, and the string after the tag is registered like the translated phrase.
for example:
"start_text$$hey! Is nice to see you again"
become in the csv:

key	en
start_text	hey! Is nice to see you again

in this way we create clean keys and at the same time we write in code the string for one of the translated languages.
To take advantage of this new functionality I have writen a singleton: string_keys_utils
this singleton have two functions;

sk("start_text$$hey! Is nice to see you again") ## return: start_text
sktr("start_text$$hey! Is nice to see you again") ## return: hey! Is nice to see you again

where is showed what they return with the csv above.
two news filters are added to take advantage of these two functions.