Use strnr() to parse hex color code

Performance compare:

gist: [gist](https://gist.github.com/UncleBill/9482014#file-gistfile2-vim)
results: https://gist.github.com/UncleBill/9482014#file-gistfile3-txt

Results:
-----------------------------------------
run: 1000 times
use eval:  0.022386
without eval:  0.019251