`fisher up` obeys $fish_path (#407)

* Ensure fisherman's completions are placed in the correct fisherman directory when running self update.

* Update documentation to include the extra steps required when setting fish_path.

* -U instead of -g in README.

README.md

@@ -139,10 +139,17 @@ The configuration and cache are saved to ~/.config/fisherman and ~/.cache/fisher
 
 The fishfile and plugins are saved to ~/.config/fish by default.
 
-To customize this location:
+To customize this location, add the following to your ~/.config/fish/config.fish file:
 
 ```fish
 set -U fish_path ~/my/path
+
+for file in $fish_path/conf.d/*.fish
+  builtin source $file ^ /dev/null
+end
+
+set fish_function_path $fish_path/functions $fish_function_path
+set fish_complete_path $fish_path/completions $fish_complete_path
 ```
 
 ### What is a fishfile and how do I use it?

fisher.fish

@@ -686,11 +686,11 @@ end
 function __fisher_self_update
     set -l file (status --current-filename)
 
-    if test "$file" != "$fish_config/functions/$fisher_cmd_name.fish"
+    if test "$file" != "$fish_path/functions/$fisher_cmd_name.fish"
         return 1
     end
 
-    set -l completions "$fish_config/completions/$fisher_cmd_name.fish"
+    set -l completions "$fish_path/completions/$fisher_cmd_name.fish"
     set -l raw_url "https://raw.githubusercontent.com/fisherman/fisherman/master/fisher.fish"
     set -l fake_qs (date "+%s")