input: Fix sizes of input/read buffers

Input buffer must be bigger than read buffer to ensure it always has space for
converted data.
api: Add vim_input function and mark vim_feedkeys as deferred

The `vim_feedkeys` must be deferred because it can potentially free the buffer
passed to `os_inchar`(which in turns calls `vim_feedkeys` indirectly).

The new `vim_input` function can be used to emulate user input(Since it does not
mess with the typeahead, it is safe to execute without deferring).
travis: Make sure valgrind report is shown on test failures