Add support for text formatting

[BernieWhite]

Currently PSDocs doesn't have any way to format text for example italic or bold without preformatting text or mashing together string literals.

Add support for formatting of text with a Format block syntax, possibly also add regex selection of words.

Example syntax:

# Text on pipeline
'Some text to bold' | Format -Style Bold

# Text in block
Format -Style Bold,Italic {
  'Some text to bold and italicise'
}

# Apply formatting only to selected regex match
'Just bold the word bold only' | Format -Match 'bold' -Style Bold