Define table and column names as arguments in a plpgsql function?

Question

It must be simple, but I'm making my first steps into Postgres functions and I can't find anything that works...

I'd like to create a function that will modify a table and / or column and I can't find the right way of specifying my tables and columns as arguments in my function.

Something like:

CREATE OR REPLACE FUNCTION foo(t table)
RETURNS void AS $$
BEGIN
   alter table t add column c1 varchar(20);
   alter table t add column c2 varchar(20);
   alter table t add column c3 varchar(20);
   alter table t add column c4 varchar(20);
END;
$$ LANGUAGE PLPGSQL;

select foo(some_table)

In another case, I'd like to have a function that alters a certain column from a certain table:

CREATE OR REPLACE FUNCTION foo(t table, c column)
RETURNS void AS $$
BEGIN
   UPDATE t SET c = "This is a test";
END;
$$ LANGUAGE PLPGSQL;

Is it possible to do that?