Add support for inputting the template from stdin

[AlexanderAmelkin]

It must be possible to run cat template | frugen --from - fru.bin and get a fru.bin generated from template.

[AlexanderAmelkin]

It turned out that json-c library prior to version 0.12 was unable to read the json data from an already opened file descriptor such as stdin, but some distributives still provide version as old as 0.11, although 0.15 is already available.

Unless a really strong demand arises for this feature, I'm not going to implement it so far as my development machine is one of those having 0.11 only.