add support for intelligent string amplification

[monperrus]

This would be an extension of http://mp.binaervarianz.de/ase2017_TestMiner.pdf

[danglotb]

Hello,

It seems great! Actually, it would start to resolve an important problem in DSpot: the number of amplified test by input amplification.

For now, most of input amplification are random strings but it seems to be quite useless.

I'll take a look more deeply ASAP.