Better custom theme loading

For better organization of custom themes, one can organize them in **custom/themes**, instead of dumping them all in the **custom** directory. However, `oh-my-zsh.sh` does not currently check that path for themes, despite it being a logical place for themes, as it mimics default oh-my-zsh directory structure.

In my small tweak, `oh-my-zsh.sh` now also checks for **$ZSH_CUSTOM/themes/$ZSH_THEME.zsh-theme** when attempting to load a theme.

For example:

    $ ls .oh-my-zsh/custom
    foo bar blah customtheme.zsh-theme anothertheme.zsh-theme
Becomes

    $ ls .oh-my-zsh/custom
    foo bar blah
    $ ls .oh-my-zsh/custom/themes
    customtheme.zsh-theme anothertheme.zsh-theme