Number of improvements

# Added "None" keyword to create files without extension after extraction
Added "None" keyword to create files without extension after extraction
I am not sure where to add "None" extension handler. 

I faced a problem when I am extracting custom firmware with {file-name} already having an extension(ex: index.html, ring.wav, binary.elf and etc).
 
Before the patch with the following extractor.conf:
"file in firmware:None"

binwalk extracted the following files:
index.html.None
ring.wav.None
binary.elf.None

after binwalk patching i have the following files:
index.html
ring.wav
binary.elf

# Support for argument math evaluation 
Changed \_getmath_arg to use evaluation of math expression. Now we can use the following magic:

\>20 lelong x {offset-adjust:100+2*%d}

# Added {string-len} smart signature macros 
Added {string-len} smart signature macros. For this macros I have the following use case:

\>20 string x {offset-adjust:21+{string-len:%s}}

# Usage example

\>4	lelong x	Length:%d-{string-len:
\>20	string x	\b%s}
\>20	string x	{file-name:%s}
\>4	lelong x	{file-size:%d-{string-len:
\>20	string x	\b%s}-1}
\>20	string x	{offset-adjust:21+{string-len:%s}}