Check ROM file contents for sanity and error out if system would be unbootable

Small commandline fix

Merge pull request #19 from rakslice/ctrl_c_without_poll

count ctrl-c presses immediately

Swap around DEL/BS inputs, some text rewordings

Link fixes

Emscripten version: dark mode, add help and about

Use sane defaults for rtc ram rather than all-zeroes

Pass ctrl-c and other signals through to emulator

Add caching for emsdk

Remove dependency on unpkg.org

Remove xterm-pty as a submodule. We will bring it in manually for the web build.

Slight readme style fixes

Change license to MIT; add note to readme about use of Musashi

Fix off-by-one-instruction in address for disassembly. Thanks Rakslice for noticing. Closes #17.