Propose an escape character for the alias

[mgoonde]

Hello,
i think it might be useful to have an escape character for the aliases. Here is an idea to use \ as the escape character, such that if |some_alias| is defined, typing \|some_alias| will escape it and write verbatim |some_alias| instead.

I defined the ALIAS_RE with a lookbehind for the \ character, then do the replacements in 2 steps: replace the non-escaped aliases with their definition; then replace the escaped aliases with their verbatim word, without the \.

What do you think?

--- a/ford/_markdown.py
+++ b/ford/_markdown.py
@@ -133,7 +133,8 @@ class AliasPreprocessor(Preprocessor):
     """Substitute text aliases of the form ``|foo|`` from a dictionary
     of aliases and their replacements"""
 
-    ALIAS_RE = re.compile(r"\|([^ ].*?[^ ]?)\|")
+    # pattern to match alias only if not preceded by `\`
+    ALIAS_RE = re.compile(r"(?<!\\)\|([^ ].*?[^ ]?)\|")
 
     def __init__(self, md: Markdown, aliases: Dict[str, str]):
         self.aliases = aliases
@@ -149,7 +150,11 @@ class AliasPreprocessor(Preprocessor):
 
     def run(self, lines: List[str]) -> List[str]:
         for line_num, line in enumerate(lines):
-            lines[line_num] = self.ALIAS_RE.sub(self._lookup, line)
+            # replace the real aliases
+            line = self.ALIAS_RE.sub(self._lookup, line)
+            # replace the escaped aliases verbatim, without the preceding escape char `\`
+            line = re.sub(r"\\(\|([^ ].*?[^ ]?)\|)", r"\g<1>",line)
+            lines[line_num]=line
         return lines

[ZedThree]

Nice! Yes, I could see this being useful. Do you want to add a test and make a PR?