Add check for GNOME’s screen reader Orca

[paulmenzel]

The name clash with GNOME screen reader Orca caused confusion for one of our users today, and it looks like other people reported it too.

Could Plotly check, if the “wrong” Orca is in the path, and give a helpful error message. Currently, the user is just confused, why Orca seems to be installed, but the binary is not accepted.

$ which orca
/usr/bin/orca

> library(plotly)
> orca(p,"test.pdf")
Error: The orca command-line utility is required for this functionality.

Please follow the installation instructions here -- https://github.com/plotly/orca#installation