[Request Feature] Support for multi strings files

[titanseason]

Now this plugin can just generate the i18n.dart for strings_*.arb, but if the project is complicated, in most cases, developers want to put strings in diffrent files, for example 'module1_.arb module2_.arb ...' , thus the strings can be well orgnized.

So, can you add this feature?

P.S. Sorry, my English is not that good, I don't know if you can understand what I mean.

[noordawod]

Leaving here for a possible future implementation.

[titanseason]

What if strings.module1_*.arb strings.module2_*.arb ...
All the .arb files are start with strings, but spilit with dot