Allow for relative path names for import statements, for instance, themes

Currently all import statements appear to be limited to absolute paths.

I use timewarrior/taskwarrior on Ubuntu linux, but also on my Mac. The home directories are slightly different so it'd be nice to allow for one config file to be used on both for ease of use and consistency. However, I seem to have to create a separate absolute path entry for my Mac then for my Ubuntu machine

```
Currently required:
# import /home/arooni/.timewarrior/dark.theme
#import /Users/arooni/.timewarrior/dark.theme

```

I have to comment out one entry depending on which platform. Relative path support would solve this