Run plugin: add memory limit

[Akuli]

On linux, it's annoying when my dumb program allocates so much memory that the entire computer freezes. This is because linux decides that it's a good idea to use swap for more memory. Swap basically means using the hard drive as if it was RAM, and that's really really slow.

To help with this, add a setting that limits how much memory the program can allocate. The setting shows up in the run dialog, but it's saved to Porcupine's settings so that it won't change if you run a different command.

[Akuli]

This might not be a great idea, because if I understand the preexec_fn warning in docs correctly, it can freeze Porcupine even when we don't run out of memory.

[Akuli]

I have a better idea for implementing a memory limit, but it will require some changes.

First, we should change no_terminal.py so that it uses bash_run.sh (except on Windows, of course). Currently bash_run.sh is invoked only when a command is ran in an external terminal.

Once that is done we can simply add a ulimit command to bash_run.sh, although it will need to take a parameter describing what memory usage to apply. It already takes one parameter indicating whether to wait for user's enter press once the command is done.

[Moosems]

Personally I think the idea of swap for RAM that's not used as often is kinda smart, I just feel like it could be implemented a bit better.