Offer to install SDKMAN! if not already installed.

Fixes issue #26

Also:

 - Change language in README to clarify that
   SDKMAN! is not included.
 - Polishes to `.editorconfig`.

functions/sdk.fish

@@ -1,9 +1,36 @@
-# Guard: SDKMAN! needs to be installed
-if not test -f "$__fish_sdkman_init"
-  exit 0
-end
-
-# Declare the sdk command for fish
 function sdk -d "Manage SDKs"
-    __fish_sdkman_run_in_bash "source \"$__fish_sdkman_noexport_init\" && sdk $argv"
+    # Guard: SDKMAN! needs to be installed
+    if not test -f "$__fish_sdkman_init"
+        # Propose to install SDKMAN!
+
+        function read_confirm
+            while true
+                read -l -P "$argv[1] [y/N] " confirm
+
+                switch $confirm
+                case Y y
+                    return 0
+                case '' N n
+                    return 1
+                end
+            end
+        end
+
+        if read_confirm "You don't seem to have SDKMAN! installed. Install now?"
+            if not which curl > /dev/null
+                echo "curl required"
+                return 1
+            end
+            if not which bash > /dev/null
+                echo "bash required"
+                return 1
+            end
+
+            curl -s "https://get.sdkman.io" | bash | sed '/All done!/q'
+            echo "Please open a new terminal/shell to load SDKMAN!"
+        end
+    else
+        # Declare the _actual_ sdk command for fish
+        __fish_sdkman_run_in_bash "source \"$__fish_sdkman_noexport_init\" && sdk $argv"
+    end
 end