Adding a "thismonth" option to the date argument for fetch_crashids

[willkg]

This adds a "thismonth" value to the value domain for the `--date" argument because it's not uncommon to be testing signature generation changes for situations that have happened in the last month, but not in the last week.

$ just shell

app@socorro:/app$ socorro-cmd fetch_crashids --date=thisweek --signature-contains=g_malloc

app@socorro:/app$ socorro-cmd fetch_crashids --date=thismonth --signature-contains=g_malloc
9c924bb4-cc8d-4071-afef-000140241031
72d845fc-6f33-4f44-9203-1dc490241104
e4a0a8dd-341b-430a-9f6a-2ad0c0241028
aef97233-ca73-4ba9-a6d1-c8a280241104
ede7036b-4927-41a0-834e-9e4a80241105
4ea1423a-fce6-45a9-95e2-6a05e0241110
f17cd077-17a7-46cd-a378-2dc8a0241102
8d1cc235-dcbd-4606-a99d-eaf840241104
2b89f970-0451-43fc-a237-ecf7a0241106

[willkg]

Thank you!