consolidate filtering and sanitization

[williballenthin]

currently, we spread the logic of filtering and sanitizating all over the place, including:

• utils: https://github.com/fireeye/flare-floss/blob/6670c059d4d2fe714cf732ad4662f0be24f31ffd/floss/utils.py#L90-L118
• main: https://github.com/fireeye/flare-floss/blob/6670c059d4d2fe714cf732ad4662f0be24f31ffd/floss/main.py#L76-L84
• main: https://github.com/fireeye/flare-floss/blob/6670c059d4d2fe714cf732ad4662f0be24f31ffd/floss/main.py#L352-L360

and we propose:

• exclude/tag common standard library strings #406 stdlib strings
• exclude/tag strings referenced by library functions (identified by FLIRT) #362 flirt strings
• filter out static strings that are found within instructions/code #405 instruction strings

we should figure out how to do this filtering in a few/one place and make it easy to understand.