Ability to parse an ELF that is loaded into memory

[eyalgolan1337]

Currently, when supplying a byte vector of an ELF file to parse, LIEF refers to it as it was the disk content of the ELF, and thus uses only the file_offset values to parse the ELF.
When supplying a byte vector of an ELF that is loaded into memory, and by telling LIEF that this ELF is from memory and not from disk, LIEF should be able to use the virtual_address values instead.