option to handle errors by breaking into a debugger (same behavior as in ipython --pdb)
this feature is invaluable for debugging:

```
 ipython3 --pdb -i tests/python/t01_inim_issue_debug_on_error.py
Python 3.6.5 (default, Jun 20 2018, 01:40:23)
Type 'copyright', 'credits' or 'license' for more information
IPython 6.1.0 -- An enhanced Interactive Python. Type '?' for help.
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
~/git_clone/nim/timn/tests/python/t01_inim_issue_debug_on_error.py in <module>()
      4   print(b[3])
      5
----> 6 fun()

~/git_clone/nim/timn/tests/python/t01_inim_issue_debug_on_error.py in fun()
      2   a = 10
      3   b = [1, 2, 3]
----> 4   print(b[3])
      5
      6 fun()

IndexError: list index out of range
> /Users/timothee/git_clone/nim/timn/tests/python/t01_inim_issue_debug_on_error.py(4)fun()
      2   a = 10
      3   b = [1, 2, 3]
----> 4   print(b[3])
      5
      6 fun()

ipdb> h

Documented commands (type help <topic>):
========================================
EOF    cl         disable  interact  next    psource  rv         unt
a      clear      display  j         p       q        s          until
alias  commands   down     jump      pdef    quit     source     up
args   condition  enable   l         pdoc    r        step       w
b      cont       exit     list      pfile   restart  tbreak     whatis
break  continue   h        ll        pinfo   return   u          where
bt     d          help     longlist  pinfo2  retval   unalias
c      debug      ignore   n         pp      run      undisplay

Miscellaneous help topics:
==========================
exec  pdb

ipdb> whatis a
<class 'int'>
ipdb> pp a
10
ipdb> pp b
[1, 2, 3]
```

tests/python/t01_inim_issue_debug_on_error.py
```nim
def fun():
  a = 10
  b = [1, 2, 3]
  print(b[3])

fun()
```

## implementation
this could actually be pretty simple:
we could use this when compiling programs:
nim --debugger:native|endb

NOTE: this option should be enabled / disabled during inim session

NOTE: more generally, options for nim compilation (cf the `--nim=` I added) should be customizable during inim session. Just need a good way to handle these "meta" commands
