chisels: Add parse_numeric_input() in common.lua

This commit adds a function in common.lua to parse numeric user input
and exit with a nice error message if something goes wrong.

This new function uses tonumber() in order to convert user input to a
number. If tonumber() returns 'nil', aka input wasn't a number, the
function prints an error message to inform the user and calls
os.exit() to stop execution.

All chisels that need to accept numeric user input should use this
function from now on.
chisels: Update chisels to use parse_numeric_input

Update various chisels to use function 'parse_numeric_input' instead of
tonumber(). The following chisels are updated:

* fileslower
* netlower
* scallslower
* spy_users
Merge pull request #249 from Erethon/master

Chisels: Add a function to parse and handle numeric user input