API for expanding Magic string

[seanshpark]

Jerry has magic string for strings that are commonly used so to save the heap, I think.
As for embedding Jerry, it would be useful to register magic strings with an API to save heap space,
for example IoT.js built-in scripts.

[seanshpark]

This is the first idea talked on CC

[seanshpark]

@egavrin , can you please look at my initial implementation?
seanshpark@56c1d8d

[egavrin]

@seanshpark briefly checked, looks good.
PS implementation will require some modifications as soon as @sand1k will upstream literal storage.

[seanshpark]

Close issue as #136 has landed