Added basic code functionality and unit tests

[fussel178]

Added

• system information wrapped from systeminformation npm module.
• options parsing to specify result by define function names from systeminformation reference and filter options (see (README.md))
• unit tests to reach 100% code coverage

Changed

nothing

Notes

• the functions si.time() and si.version() are non promise function but do not break the await statement in the code. Interesting

[pklaschka]

Looks good so far, just the JSDoc comment is a bit off (see my inline-comment regarding that). Other than that, I think it's good (more on that later this evening, I didn't have enough time right now)