Symlink files by convention

Files ending with '.symlink' and '.<os>-symlink' will be symlinked to
$HOME.

If a file or folder needs to be symlinked to from a different location
depending on the OS, a config file should be added as
'<filename>.symlinks'. The content of the file should have the following
structure:

    <os1>: path/to/file1
    <os2>: path/to/file2
    ...

See 'vim/.vim.symlinks' for an example.