feat: add basic markdown support

[camshaft]

Fixes #9

Description of changes:

This change adds support for citing markdown specifications. I've added a bunch of new tests to make sure nothing broke in the process.

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[seebees]

Using https://github.com/awslabs/aws-encryption-sdk-specification/blob/master/framework/aws-kms/aws-kms-key-arn.md as an example Markdown.

On extract the -f MARKDOWN MUST be passed.
However, with the change added,
it can recognize the markdown.
But, that's fine :)

However, the requirements parsing is off by "1".

The above produces the following extracted TOML

target = "aws-encryption-sdk-specification/framework/aws-kms/aws-kms-key-arn.md#appendix-A valid AWS KMS ARN"

# A valid AWS KMS ARN
#
# 
# A string with 5 `:` that delimit following 6 parts:
# 
# 1. MUST start with string `arn`
# 2. The partition MUST be a non-empty
# 3. The service MUST be the string `kms`
# 4. The region MUST be a non-empty string
# 5. The account MUST be a non-empty string
# 6. The resource section MUST be non-empty and MUST be split by a single `/`
#    any additional `/` are included in the resource id
#    1. The resource type MUST be either `alias` or `key`
#    2. The resource id MUST be a non-empty string
# 

[[spec]]
level = "MUST"
quote = '''
MUST start with string `arn`
2.
'''

[[spec]]
level = "MUST"
quote = '''
The partition MUST be a non-empty
3.
'''

[[spec]]
level = "MUST"
quote = '''
The service MUST be the string `kms`
4.
'''

[[spec]]
level = "MUST"
quote = '''
The region MUST be a non-empty string
5.
'''

[[spec]]
level = "MUST"
quote = '''
The account MUST be a non-empty string
6.
'''

[[spec]]
level = "MUST"
quote = '''
The resource section MUST be non-empty and MUST be split by a single `/`
any additional `/` are included in the resource id
1.
'''

[[spec]]
level = "MUST"
quote = '''
The resource section MUST be non-empty and MUST be split by a single `/`
any additional `/` are included in the resource id
1.
'''

[[spec]]
level = "MUST"
quote = '''
The resource type MUST be either `alias` or `key`
2.
'''

[[spec]]
level = "MUST"
quote = '''
The resource id MUST be a non-empty string
'''

[camshaft]

The parser is missing list handling so it's assuming the number is part of the sentence on the previous line.

[seebees]

There is some kind of corner case with Appendix in a title

/framework/caching-cmm/appendix-a-cache-entry-identifier-formulas.toml#0:0 - section "a-cache-entry-identifier-formulas" not found in "aws-encryption-sdk-specification/framework/caching-cmm.md"
source errors were found. no reports were generated

https://github.com/awslabs/aws-encryption-sdk-specification/blob/master/framework/caching-cmm.md#appendix-a-cache-entry-identifier-formulas

[seebees]

In the report, the whole line is not highlighted.

[seebees]

These look good to me.
The ESDK specification parses now with Markdown.