is it possible to extend vim functionality via custom extension (preferably, written in Python)?
What I need ideally is custom command when in command mode. E.g.
vim supports scripting in python (and in perl as well, I think).
You just have to make sure that the vim distribution you are using has been compiled with python support.
If you are using a Linux system, you can download the source and then compile it with
./configure --enable-pythoninterp make sudo make install
Inside vim, you can type
to list the available features; if it has python support, you should see a '+python' somewhere (a '-python' otherwise).
Then, to check the usage of the python module, you can type
P.S: if you're going to compile the vim sources, make sure to check the available configure options, you might need to specify --with-python-config-dir as well.
P.P.S: to create a "custom command in command mode" (if I understand correctly what you mean), you can create a function "MyFunction" in a vim script (using python or the vim scripting language) and then invoke it with