Split into fragments only requested files

[dmdashenkov]

The current implementation fragmentizes all the files under code_root which match code_includes patterns. We can do less work by looking at which files are actually mentioned in <?embed-code?> instructions and only fragmentizing those files.

[alexander-yevsyukov]

As my work on embedding the code into pages of spine.io show, the performance impact of this issue is significant. IDEA has to chew over 43,000 file changes which is painfully slow.