Unit prefix support

[jan-kuechler]

Add support for unit prefixes, like nano, kilo, etc..

Developer Note:
Change the symbol -> rule processing to the following procedure:

1. Search the normal way, with the full symbol
2. If no rule is found:
   2.1 Extract the first character of the symbol
   2.2 If it is no valid prefix, bail out
   2.3 Otherwise search the rule for the rest-symbol
   2.4 And apply the prefix value to the factor