when gisting stdin default to filename 'gistfile1.txt' instead of '-'

fixes #235

test plan:
```
  - ensure you're signed in (ie. have a token)

  % gist Gemfile
  # check gist, filename should be 'Gemfile'

  % jot 5 | gist
  # check gist, filename should be 'gistfile1.txt'

  % jot 5 | gist Gemfile -
  # check gist, filenames should be 'Gemfile' and 'gistfile1.txt'
```

Author: Philip Hallstrom

lib/gist.rb

@@ -76,10 +76,14 @@ def auth_token
   #
   # @see http://developer.github.com/v3/gists/
   def gist(content, options = {})
-    filename = options[:filename] || "a.rb"
+    filename = options[:filename] || default_filename
     multi_gist({filename => content}, options)
   end
 
+  def default_filename
+    "gistfile1.txt"
+  end
+
   # Upload a gist to https://gist.github.com
   #
   # @param [Hash] files  the code you'd like to gist: filename => content
@@ -114,7 +118,8 @@ def multi_gist(files, options={})
       if content.to_s.strip == ""
         raise "Cannot gist empty files" unless options[:skip_empty]
       else
-        json[:files][File.basename(name)] = {:content => content}
+        name = name == "-" ? default_filename : File.basename(name)
+        json[:files][name] = {:content => content}
       end
     end