Make dir levels shown in the prompt configurable

Until now the prompt only shows the last three levels of the prompt,
however I'd personally prefer to have some more levels (i.e. five).

I figured that it's not worth debating about how many levels make sense,
instead it's now configurable in `~/.zshrc`, the current default will be
kept.

This can be achieved by adding the following line to `~/.zshrc` *before*
loading this theme:

```
export LAMBDA_MOD_N_DIR_LEVELS=5
```

README.md

@@ -7,3 +7,15 @@ A ZSH theme optimized for people who use:
 ---
 
 ![Screenshot](https://raw.githubusercontent.com/halfo/lambda-mod-zsh-theme/master/screenshot.png)
+
+## Configuring the amount of dir levels
+
+By default only the last three directories are shown in the prompt. The amount of
+levels can be configured by setting `LAMBDA_MOD_N_DIR_LEVELS` before loading the prompt:
+
+``` zsh
+# ~/.zshrc
+export LAMBDA_MOD_N_DIR_LEVELS=10
+
+# load `lambda-mod` and `oh-my-zsh`
+```

lambda-mod.zsh-theme

@@ -30,7 +30,7 @@ function get_right_prompt() {
 
 PROMPT=$'\n'$LAMBDA'\
  %{$fg_bold[$USERCOLOR]%}%n\
- %{$fg_no_bold[magenta]%}[%3~]\
+ %{$fg_no_bold[magenta]%}[%'${LAMBDA_MOD_N_DIR_LEVELS:-3}'~]\
  $(check_git_prompt_info)\
 %{$reset_color%}'