Extend the ~m pattern to allow relative ranges

Extend the ~m pattern to allow relative ranges

The ~m pattern now accepts absolute ranges, relative ranges and has some
mnemonics (shortcuts).

There are shortcuts for some emails:
    ^   First email
    .   Current email
    $   Last email

These shortcuts can be used anywhere a number is used in the examples
below.

Relative ranges have a comma ',' in them:
    -3,.    Previous 3 emails plus the current one
    .,$     Current email and all the later ones
    -2,2    Previous 2, current and next 2 emails
    0,1     Current and next email

Absolute range:
    3-10    Emails 3-10
    -10     Emails 1-10
    10-     Emails 10-last
    7       Just email 7

Closes: #269
Closes: #283