Introduced few new commands to handle breakpoints:
- `b file:NNN` to set new breakpoint;
- `bd file:NNN` to remove breakpoint;
- 'bl` to list all active breakpoints.

There is partial breakpoint notation supported when
for `b :N` or `b +N` breakpoint will be set to the
asked line in the _currently_ debugged file.

Changed `c` (continue) to stop on breakpoints, if there any active
one asssigned by user. Otherwise it still run in full speed, without
any hook check.

NO_DOC=see the later commit
NO_CHANGELOG=see the later commit