Allow symlink to define OS-specific filenames

[alextercete]

This is to allow Vim's folder (.vim on Linux/OSX; vimfiles on Windows) to be handled by the convention strategy.

The convention would need to change from <filename>.{<os>-}symlink to {[<os>]}<filename>.symlink.

Given the following folder structure:

`-- vim
    |-- .vim;[windows]vimfiles.symlink
    |   `-- [...]
    |-- .gvimrc.symlink
    |-- .vimrc.symlink
    |-- [windows]_gvimrc.symlink
    |-- [windows]_vimrc.symlink
    `-- [windows]_vsvimrc.symlink

Then the following symbolic links should be created:

On Windows

~/.vim -> path/to/vim/.vim;[windows]vimfiles.symlink
~/vimfiles -> path/to/vim/.vim;[windows]vimfiles.symlink
~/.gvimrc -> path/to/vim/.gvimrc.symlink
~/.vimrc -> path/to/vim/.vimrc.symlink
~/_gvimrc -> path/to/vim/[windows]_gvimrc.symlink
~/_vimrc -> path/to/vim/[windows]_vimrc.symlink
~/_vsvimrc -> path/to/vim/[windows]_vsvimrc.symlink

Notice how both .vim and vimfiles are created in this case.

On other platforms

~/.vim -> path/to/vim/.vim;[windows]vimfiles.symlink
~/.gvimrc -> path/to/vim/.gvimrc.symlink
~/.vimrc -> path/to/vim/.vimrc.symlink

Multiple OS's

Multiple OS's can be specified:

```text
`-- vim
    `-- [osx,linux].vim;[windows]vimfiles.symlink

Which would lead to .vim only being created on OSX and Linux.

Excluded OS's

Exclusions can be specified:

```text
`-- vim
    `-- [-windows].vim;[windows]vimfiles.symlink

Which would lead to .vim being created on all OS's but Windows.