Vertical scrolling & rustfmt changes

Completely prevents the cursor from walking offscreen.
Also fixes several crashes when pressing 'z'. I still don't
understand how those are supposed to work anyway, but allowing
the cursor to go offscreen is never a good idea.

Implement 'I', 'A' and 'c' commands.

Also fixes a bug where pressing 'L' would put
the cursor beyond the last character of the line.

Fix a panic when using 'dL' or 'dG'

Fix Sodum logo in README

Seems like the path to the Sodium logo changed.

Fix a bug in insert_line

Assuming that insert_line(n) shall insert the string
into nth-line (0 based).
- Fix a bug in insert_line when insert into after buffer
- Do corresponding changes in other place

Add exit key

Fix compilation errors on the generic branch

Fix a panic when doing 0g

Fix the crash when deleting a cursor

Update README.md

The new behaviour is to go to the end of the document (as with vim)

Rewrite argument handling, add read-only mode

Fixed typo.

Allows for chained short options (e.g. -Clx)

Use help.txt as help message text.

Add initial command line arguments.

adds the W command to go to next WORD

Add support for --version, --help/-h and -R.
-R is without function as of now.

fixing mixup of h and l movement

updating orbclient crate version in Cargo.toml