Adding variable to define Custom browser to use (#18)

Added custom variable `grip-url-browser` to specify another browser to
be used instead of default one, and `grip-url-args` to pass to 
`grip-url-browser`.

Added function grip--browser which will load preview either by using
grip-url-browser or default browser

README.md

@@ -76,6 +76,14 @@ Run `M-x customize-group RET grip RET` or set the variables.
 ;; Path to the grip binary
 (setq grip-binary-path "/path/to/grip")
 
+;; You can use this variable to define another browser
+;; to use when loading previews. By default this value is `nil`
+;; meaning use default browser defined by your system
+(setq grip-url-browser "custom_browser")
+
+;; If you want to pass arguements to your custom browser then use
+(setq grip-url-args '("arg1" "arg2" "etc"))
+
 ;; A GitHub username for API authentication
 (setq grip-github-user "")
 

grip-mode.el

@@ -56,6 +56,16 @@
   :type 'file
   :group 'grip)
 
+(defcustom grip-url-browser nil
+  "Browser to launch Markdown/Org previews.
+Use default browser if nil."
+  :type '(choice (const :tag "None" nil) string)
+  :group 'grip)
+
+(defcustom grip-url-args nil
+  "A list of strings defining options for `grip-url-browser'."
+  :type '(repeat (string :tag "Argument")))
+
 (defcustom grip-github-user ""
   "A GitHub username for API authentication."
   :type 'string
@@ -81,7 +91,6 @@ option."
   :type 'boolean
   :group 'grip)
 
-
 
 ;; Externals
 (declare-function xwidget-buffer 'xwidget)
@@ -98,6 +107,17 @@ option."
 (defvar-local grip--preview-file nil
   "The preview file for grip process.")
 
+(defun grip--browser (url)
+  "Use browser specified by user to load URL.
+Use default browser if nil."
+  (if grip-url-browser
+      (let ((browse-url-generic-program grip-url-browser)
+            (browse-url-generic-args grip-url-args))
+        (ignore browse-url-generic-program)
+        (ignore browse-url-generic-args)
+        (browse-url-generic url))
+    (browse-url url)))
+
 (defun grip--browse-url (url)
   "Ask the browser to load URL.
 
@@ -110,7 +130,7 @@ Use default browser unless `xwidget' is available."
           (when (buffer-live-p buf)
             (and (eq buf (current-buffer)) (quit-window))
             (pop-to-buffer buf))))
-    (browse-url url)))
+    (grip--browser url)))
 
 (defun grip--preview-url ()
   "Return grip preview url."