Added functions concat, minify, replace, and more

I have implemented the following functions (as discussed in issue #2):

- concat
- minify (using uglify-js)
- replace
- and some more utility functions: read, write, getToday, getPackage, getVersion

I have implemented the functions as globals to be consistent with the library, but I think it would be a good idea not to use globals but a regular `exports` instead.