Enable use of SI unit abbreviations

[gcomte]

Allow user input 1k to be translated into 1,000 and 1M into 1,000,000 and so on (check out this list)

Fixes #152

[raweber42]

Looks good. Maybe there is a less verbose way to read the amount so that you don't need to read it as a string and strip away the last character, but I don't know what Rust provides for this :)

[gcomte]

Looks good. Maybe there is a less verbose way to read the amount so that you don't need to read it as a string and strip away the last character, but I don't know what Rust provides for this :)

Great input but for now I fail to come up with a better scheme.