In-toto record functionality

[shibumi]

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Description of issue or feature request:
Most of the run functionality has been fulfilled by #56, what is missing is record functionality for in-toto.
Record functionality is explained here: https://github.com/in-toto/in-toto#in-toto-record

in-toto-record works similar to in-toto-run but can be used for multi-part software supply chain steps, i.e. steps that are not carried out by a single command. Use in-toto-record start ... to create a preliminary link file that only records the materials, then run the commands of that step or edit files manually and finally use in-toto-record stop ... to record the products and generate the actual link metadata file. For a detailed list of all command line arguments and their usage, run in-toto-record start --help or in-toto-record stop --help, or look at the online documentation.

The difference to the python implementation is, that we do not want a cli for in-toto-golang. The functionality should be implemented as library with RecordStart and RecordStop methods.

Current behavior:
No record functionality

Expected behavior:
In-toto-golang should support record functionality for satisfying the in-toto specification.

[mikhailswift]

This is implemented with #119

[adityasaky]

Thanks for your work, @mikhailswift!