Fixes #30: String interpretation issues

[lerouquin]

Added a check for plain strings (single or double quoted).

This also resolves the following issue:

Before :

{"my string.with a dot"|myFunction}
outputs to
<?php echo myFunction( my string["with a dot"] ); ?>
which results in a parse error.

After :

{"my string.with a dot"|myFunction}
outputs to
<?php echo myFunction( "my string.with a dot" ); ?>
which is the expected behavior.

[eliemichel]

Please comment the regex to explain what it is intended to match!

1. Is it useful to "mute" parenthesis with ?: here?
2. \\\' is not a valid escape sequence. If you want to match \', you need to add one more backslash.
3. What is I want to end a string with a single backslash? "foo\" will fail since the backslash escapes the colon. But "foo\\" will display an extra backslash.

#"((\\(\\|"))|[^\\"])*?"# should work to match a double colon string. (escape it again to include it inside a PHP string).