Implemented: force python 2 or 3 with ability to switch during runtime

Simultaneous python2 and python3 support, switchable during runtime
calling a function as well as setting a variable in .vimrc! Closes
issues #200 and #210 (which gave the idea for this commit, thank you
@Shtucer).

doc/jedi-vim.txt

@@ -38,11 +38,11 @@ Contents				*jedi-vim-contents*
     6.8. squelch_py_warning		|g:jedi#squelch_py_warning|
     6.9. completions_enable		|g:jedi#completions_enable|
     6.10. use_splits_not_buffers	|g:jedi#use_splits_not_buffers|
+    6.11. force_pycmd                   |g:jedi#force_pycmd|
 7. Testing				|jedi-vim-testing|
 8. Contributing				|jedi-vim-contributing|
 9. License				|jedi-vim-license|
 
-
 ==============================================================================
 1. Introduction				*jedi-vim-introduction*
 
@@ -404,6 +404,25 @@ direction which you want to open a split with.
 Options: top, left, right or bottom
 Default: "" (not enabled by default)
 
+------------------------------------------------------------------------------
+6.11. `g:jedi#force_pycmd`                *g:jedi#force_pycmd*
+
+If you have installed both python 2 and python 3, you can force which one jedi
+should use by setting this variable. It forces the internal Vim command, which
+will be used for every jedi call to the respective python interpreter.
+The variable can be set in the .vimrc like this:
+
+let g:jedi#force_pycmd = 'python3'
+
+This variable can be switched during runtime using the following function:
+Function: `jedi#force_pycmd_switch()`
+
+or set directly using this function, which has the same name as the variable:
+Function: `jedi#force_pycmd(pycmd)`
+
+Options: 'python' or 'python3'
+Default: 'python'
+
 ==============================================================================
 7. Testing				*jedi-vim-testing*