Implement new functions of NVIM API v8

[esensar]

Implement new functions added in NVIM API v8. These functions can be discovered using API Explorer from this project. All of this belongs to neovim-api module and wrappers need to be made in neovim-rx-api module for RxJava support.

Functions:

• nvim_del_mark - https://neovim.io/doc/user/api.html#nvim_del_mark()
• nvim_get_mark - https://neovim.io/doc/user/api.html#nvim_get_mark()
• nvim_eval_statusline - https://neovim.io/doc/user/api.html#nvim_eval_statusline()

Methods:

• nvim_buf_del_mark - https://neovim.io/doc/user/api.html#nvim_buf_del_mark()
• nvim_buf_set_mark - https://neovim.io/doc/user/api.html#nvim_buf_set_mark()