Add -Zgraphviz_dark_mode and monospace font fix
Many developers use a dark theme with editors and IDEs, but this
typically doesn't extend to graphviz output.
When I bring up a MIR graphviz document, the white background is
strikingly bright. This new option changes the colors used for graphviz
output to work better in dark-themed UIs.
<img width="1305" alt="Screen Shot 2020-09-09 at 3 00 31 PM" src="https://user-images.githubusercontent.com/
3827298/
92659478-
4b9bff00-f2ad-11ea-8894-
b40d3a873cb9.png">
Also fixed the monospace font for common graphviz renders (e.g., VS Code extensions), as described in https://github.com/rust-lang/rust/pull/76500#issuecomment-
689837948
**Before:**
<img width="943" alt="Screen Shot 2020-09-09 at 2 48 44 PM" src="https://user-images.githubusercontent.com/
3827298/
92658939-
47231680-f2ac-11ea-97ac-
96727e4dd622.png">
**Now with fix:**
<img width="943" alt="Screen Shot 2020-09-09 at 2 49 02 PM" src="https://user-images.githubusercontent.com/
3827298/
92658959-
51451500-f2ac-11ea-9aae-
de982d466d6a.png">