Add a script to diff --metrics output

[kripken]

Example usage:

$ wasm-opt a.wasm --metrics > a
$ wasm-opt b.wasm --metrics > b
$ python scripts/diff_metrics a b
Const      :     +1613
RefNull    :      +114
StringConst:        -1
StructNew  :        -2
[exports]  :       +70
[funcs]    :      +132
[globals]  :        +7
[imports]  :        +4
[memories] :        +1
[tables]   :        +1
[tags]     :        +1
[total]    :     +9773
[vars]     :      +554